mid-15c., "a monkey," also "an impertinent, conceited fellow, an absurd fop," a general term of reproach (in mid-15c. especially a contemptuous nickname for William de la Pole, Duke of Suffolk), of unknown origin. Apparently from Jack of Naples, but whether this is some specific personification of Jack (which is attested from 16c. as "saucy or impertinent fellow") or folk etymology of jack (n.) + ape (n.) is unknown. See extensive note in OED. Century Dictionary suggests "orig., it is supposed, a man who exhibited performing apes." Farmer and Henley ("Slang and Its Analogues") say "originally, no doubt, a gaudy-suited and performing ape." Its fem. counterpart is Jane-of-apes (Massinger) "a pert, forward girl."