English river name, from Celtic abona "river," from *ab- "water" (see afanc). Of the at least four rivers in England and two in Scotland that bear the name, Shakespeare's is the Warwickshire Avon.

1550s, "affirm, acknowledge openly;" 1590s, "make good, answer for," from Middle 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.

"of or pertaining to an uncle," 1789, from Latin avunculus "maternal uncle," diminutive of avus (see uncle) + -ar. Used humorously for "of a pawnbroker" (uncle was slang for "pawnbroker" from c. 1600 through 19c.).

Being in genteel society, we would not, of course, hint that any one of our readers can remember so very low and humiliating a thing as the first visit to "my Uncle"—the first pawnbroker. We have been assured though, by those whose necessities have sometimes compelled them to resort, for assistance, to their avuncular relation, that the first visit—the primary pawning—can never be forgotten. [Household Words, May 15, 1852]