mid-15c., earlier staloun (c.1300), "male horse kept for breeding purposes," from Anglo-French estaloun, Old French estalon "stallion, uncastrated male horse" (Modern French étalon), from Frankish *stal, cognate with Old High German stal "stable," from Proto-Germanic *stalla- (see stall, n.1). The notion is probably of a horse kept in a stable to service mares. Transferred sense of "robustly lascivious man" is attested from 1550s.