Green’s Dictionary of Slang

thick ’un n.

also thick one
[the dimensions of the coin/foodstuff]

1. [mid-19C–1950s] a sovereign (£1).

2. [1900s] five shillings, a crown (25p).

3. [1900s–40s] a slice of bread and butter.

4. [1930s] a silver dollar.

In phrases

do someone a thick ’un (v.)

[late 19C-1930s] to play a ‘dirty trick’ on someone.

half-thick (’un) (n.) (also half-a-thick)

[late 19C–1900s] (N.Z.) a half-sovereign, ten shillings.