Green’s Dictionary of Slang

jack n.4

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.

In compounds

jack spinner (n.)

[1920s] (Aus. und.) one who operates a game of two-up .

In phrases

half-jack (n.)

1. [mid–late 19C] 10 shillings (50p), half a sovereign .

2. see sense 3 above.

make one’s jack (v.)

[late 18C–1900s] (US) to prosper, to make one’s fortune.