To go nap. To stake all the winnings on the cards in hand; hence, to risk all on one venture. Nap is a game of cards; so called from Napoleon III.
(A), a doze or short sleep, as “To take a nap,” is the Anglo-Saxon hnæppian or hnapp-ian (to take a nap; the nap of cloth is the Anglo-Saxon hnoppa.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894