Of course an attorney would help. First to use in interstate commerce, not first to apply for registration is the criterion for establishing rights. You can try to represent yourself, but against the law firm of a "large company" it won't be a fair fight.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Should [you] seek an attorney? YES.
At this point, you need to consult with a trademark attorney about the specific facts of your case. For example, the first party to use the mark in interstate commerce is a critical issue. If you have not used the mark (for example, you only filed an intent-to-use, or ITU, application), and the other company has used the mark in commerce--then the other company may have superior rights. Then again, maybe the mark is generic, so no one should have trademark rights.
This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.
The answer really depends upon how much you wish to use this slang word as your trademark, and how you want to spend of your own money to fight for it.
As Attorney Juo indicates, if you have not yet used the mark and only filed an intent-to-use application whereas the large company has already been using the mark in commerce, you will likely lose this fight.
If you know nothing about trademark law, it would certainly be worth your while to retain a trademark attorney to explain the basics to you.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.
My colleagues have already provided you with excellent answers to your questions ... walk away? (depends); seek an attorney? (yes); something i can do? (yes -- but, what you should do is dependent upon additional facts, starting with  how much you use this mark in commerce, i.e., how important is the mark to you). It has already been pointed out the difference between an intent-to-use application versus actual use. But, if you're actually using it, been using it, people have associated your company with the word, then you may wish to keep it. Also,
(2) What does the cease and desist say? Is the company actually using this mark? Have they been for years, or are they "planning" to launch a new product and scorching the earth in front of them? If they haven't used it or if they just applied like you have, perhaps you move to cancel their application. Of course, additional facts would have to be known. Also,
(3) What is your litigation budget? It is rare that a trademark case (rather in district court or in front of the USPTO) is taken on a contingency basis, so you will be looking at an hourly fight. This is the way big companies literally silence competition ... as a great lawyer once said, "in this country, you're innocent until proven broke." But,
(4) In all seriousness, there are many many questions to be answered in determining how to respond to a cease and desist. You should speak to an attorney in order to determine what is best for your business ... be mindful though ... this is a business decision / not a personal one just because you're angry about getting a cease and desist. Best of luck to you.
What is the $ value of the mark? Is the slang word made up and fanciful, or is it in a dictionary?
Curt Harrington Patent & Tax Law Attorney Certified Tax Specialist by the California Board of Legal Specialization PATENTAX.COM This communication is general information and not legal advice, and does not create an attorney-client relationship. This communication should not be relied upon as any type of legal advice. Please note that no attorney-client relationship exists between the sender and the recipient of this message in the absence of either (1) a signed fee contract and (2) remission of an agreed-upon retainer. Absent such an agreement and retainer, I am not engaged by you as an attorney, nor is any other member of my law firm.
Attorney Elman's point that the filing date of an intent-to-use trademark registration application trumps another's actual use in commerce date for trademark registration purposes should not be squirreled away in just a comment.
Quoting Gerry's comment: "If the large company started using the mark in commerce only AFTER you filed a federal trademark application on an intent-to-use basis, then you could have superior rights provided that you start actually using the mark and get a registration. On the other hand, if, as the respondents to your question seem to assume, the large company has been using the mark nationwide since BEFORE you filed the trademark application, then they would have superior rights. Yes, you should consult a trademark attorney pronto."
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
If you filed your trademark application before the company did, you may have rights in the trademark (i.e., the slang word) that are superior to that of the company. It really depends on whether you've been using the mark in commerce, and for how long, and whether you have been using it longer than the company has.
If you didn't have a trademark lawyer do the initial application to the PTO but decided to DIY, now is the time to sit down with counsel and see (a) whether the proposed mark is protectable, (b) who has superior rights, and (c) whether concurrent regional use is possible based on prior use. There are a whole lot of unknown variables here that any competent trademark attorney should be able to sort out for you.
You've received a ton of great answers already (perhaps a record) which I think is a tribute to the manner in which you have asked the question. I think you asked this fairly common question in a very appropriate manner for this forum - enough facts for us to understand your situation but most importantly omitting any identifiable facts and terms to avoid recognition possibly by your potential adversary.
As the others have said, you definitely should consult with a trademark attorney about your options unless you conclude that your level of time/investment in this mark is not worth more time and investment given your present business plans. There are numerous facts to consider which present numerous options:
1) There are about a dozen different factors alone which just deterimine whether or not the two marks may be likely to be confused and therefore infringe each other;
2) There are a whole host of factors concerning who may have prior rights in the mark - filing for registration is not one of them unless both parties rights are merely based upon an intent to use application on file in the USPTO;
3) All of the various business considerations regarding the costs and values of the mark to your business.
4) The various possible scenarios between forfeiting the mark, settling with the other party and litigation.
I believe an experienced trademark attorney can best lay out the groundwork for your situation with a decent initial consultation. I also think that a trademark attorney is better to consult initially for this than a litigation attorney because trademark law is fairly specialized and nuanced. The attorney needs to be more familiar with those specific details of trademark law and common trademark conflict scenarios than with litigation tactics which at this point could be less than 50% likely to be necessary.
Alex Butterman is a trademark attorney with Staas& Halsey LLP (http://www.staasandhalsey.com), a Washington, D.C. IP boutique law firm. Alex is admitted to the bars of Washington, D.C., New York and New Jersey but, unless otherwise specified, the answer is intended to be general enough to apply to any U.S. state and based primarily upon his knowledge and experience with applicable federal laws. The opinions expressed are those of the author and do not necessarily reflect the views of his firm, Avvo or other attorneys. This answer is for general information purposes only and is not intended to be and should not be taken as legal advice. No attoreny-client relationship or obligations are established herein, although consulting an attorney to discuss your specific situation is strongly recommended. This is especially true of trademark law and probably any area of intellectual property law because TM law is so fact-specific and full of esoteric nuances and exceptions, that more often than not there does seem to be harm in handling a trademark matter without consulting an experienced trademark attorney.
If you have received a cease and desist letter, then you definitely need to retain legal counsel. Trademark rights arise from use in commerce, not registration. If you can show that you began using the trademark in commerce before this company, and that the company's use of the trademark could cause consumer confusion, you might prevail. But if this company began using the trademark in its business before you, it will have the upper-hand. Registration is not the determining factor. What matters is who began using the trademark first, the extent to which such use resulting in "secondary meaning" among consumers such that the trademark became well-known or famous, and whether consumer confusion might result if both parties use the trademark. These are complex and sophisticated matters, and you need to work with experienced counsel to handle this correctly.