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.
well-beloved
well-born
well-bred
well-done
well-earned
well-endowed
well-fed
well-founded
well-heeled
well-hung
well-informed
