1. [late 17C–mid-19C] a farthing.
2. [19C+] (US) money.
3. [mid-19C] (also half-jack) a counter, similar in size and shape to a sovereign or half-sovereign, used in gambling houses and casinos [strangely enough, jack and half-jack are gambling counters worth a whole/half a sovereign, and half-jack (below) contemporaneously means a half-sovereign, but jack alone never seems to equal a whole sovereign].
4. [1920s] (US black) a combination of five digits used in numbers, the n.
5. [20C+] (Aus.) in two-up, a double-headed coin.
6. see jack’s (alive) n.
[1920s] (Aus. und.) one who operates a game of two-up .
1. [mid–late 19C] 10 shillings (50p), half a sovereign .
2. see sense 3 above.
[late 18C–1900s] (US) to prosper, to make one’s fortune.