Yours, Etc.: Origins and Uses of 8 Sign-Offs
1680s, "with ample material endowments," from well (adv.) + past participle of endow. Sexual sense is attested from 1951.
late 14c., indowen "provide an income for," from Anglo-French endover, from en- "in" + Old French douer "endow," from Latin dotare "bestow" (see dowry). Related: Endowed; endowing.