1610s, in male genital sense, from well (adv.) + hung (adj.).
Entries linking to well-hung
well adv.
"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.
hung adj.
"attached so as to hang down, suspended in air," past-participle adjective from hang (v.). Meaning "furnished with hangings" is from 1640s; meaning "having (impressive) male genitals" is from 1640s, originally often of animals; of a jury, "unable to agree," 1838, American English. Hung-over (also hungover) in the drinking sense is from 1950 (see hangover). Hung-up is from 1878 as "delayed;" by 1961 as "obsessed."