avow v.

mid-14c., "one whose profession is to plead cases in a court of justice," a technical term from Roman law, from Old French avocat "barrister, advocate, spokesman," from Latin advocatus "one called to aid (another); a pleader (on one's behalf), advocate," noun use of past participle of advocare "to call (as witness or adviser), summon, invite; call to aid; invoke," from ad "to" (see ad-) + vocare "to call," which is related to vox (genitive vocis) "voice" (from PIE root *wekw- "to speak").

Also in Middle English as "one who intercedes for another," and "protector, champion, patron." Feminine forms advocatess, advocatrice were in use in 15c.; advocatrix is from 17c. Old English glossed Latin advocatus with þingere (see thing).

1550s, "affirm, acknowledge openly;" 1590s, "make good, answer for," from French avochier "call upon as authority," in Old French "call (to court), advocate, plead (a case)," from Latin advocare "call to" as a witness (see advocate (n.)).

Avouch, which is no longer in common use, means guarantee, solemnly aver, prove by assertion, maintain the truth or existence of, vouch for .... Avow means own publicly to, make no secret of, not shrink from admitting, acknowledge one's responsibility for .... Vouch is now common only in the phrase vouch for, which has taken the place of avouch in ordinary use, & means pledge one's word for .... [Fowler]

Related: Avouched; avouching.