Green’s Dictionary of Slang

slug n.1

[SE slug, a piece of lead (which may ult. refer to SE slug, the gastropod; SAmE slug, the name of various large gold coins issued privately in California c.1850, usu. worth $50]

1. as a projectile.

(a) [18C+] a bullet.

(b) [1930s] a shell.

2. in senses of drink and drugs.

(a) [mid-18C] a fiery drink.

(b) [mid-18C+] a portion or measure of liquor.

(c) [1940s+] a portion of a non-alcoholic drink, e.g. coffee.

(d) [1950s] in non-alcoholic contexts, a portion, a share.

(e) [1960s] a portion or measure of a drug.

3. as a coin or token.

(a) [mid-19C+] (US) $1; thus half a slug, fifty cents; thus money, irrespective of amount.

(b) [1910s+] a token.

In phrases

chuck a slug (v.) (also toss a slug)

[1930s] to shoot.

fire a slug (v.)

[late 18C–early 19C] to drink a dram.