1680s, "with ample material endowments," from well (adv.) + past participle of endow (v.). Sexual sense is attested from 1951. A Middle English term for "naturally well-endowed" was furnished in nature.
"in a satisfactory manner," Old English wel "abundantly, very, very much; indeed, to be sure; with good reason; nearly, for the most part," from Proto-Germanic *wel- (source also of Old Saxon wela, Old Norse vel, Old Frisian wel, Dutch wel, Old High German wela, German wohl, Gothic waila "well"), from PIE root *wel- (2) "to wish, will" (source also of Sanskrit prati varam "at will," Old Church Slavonic vole "well," Welsh gwell "better," Latin velle "to wish, will," Old English willan "to wish;" see will (v.)).
Also used in Old English as an interjection and an expression of surprise. The adjective was in Old English in the sense "in good fortune, happy," from the adverb; sense of "satisfactory" is from late 14c.; "agreeable to wish or desire" is from mid-15c.; "in good health, not ailing" is from 1550s. Well-to-do "prosperous" is recorded by 1794.
late 14c., indowen "provide an income for," from Anglo-French endover, from en- "in" (see en- (1)) + Old French douer "endow," from Latin dotare "to endow, bestow, portion," from dos (genitive dotis) "marriage portion," from PIE *do-ti, from root *do- "to give." Related: Endowed; endowing.
<a href="https://www.etymonline.com/word/well-endowed">Etymology of well-endowed by etymonline</a>
Harper, D. (n.d.). Etymology of well-endowed. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/well-endowed