My ex wife just served me papers for law suit of 21,000 for money she claims I borrowed with no agreement or signed contract

Oral agreements are enforceable in CA, but a breach of oral contract lawsuit has to be filed within 2 years from the date of breach, so one defense you may have is this statute of limitations.

She has the burden of proving that there was this oral agreement for this loan, and that you rbached the agreement by not paying it back. Oral contracts are more difficult to prove than written ones, because she'll have to use her bank records, her own testimony, other witnesses if any, emails, text messages, etc. that support her version of the facts.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.