Green’s Dictionary of Slang

bull n.3

[abbr. bull’s eye n. ]

1. [late 18C–1910s] five shillings, a crown; thus demi-bull,, 2/6.

2. [mid-19C] (Aus.) 75 strokes of the lash [each number of punitory lashes was named for a different value of coin].

3. [1910s] a counterfeit coin.

In phrases

half-a-bull (n.) (also half-bull)

[late 18C–1900s] half-a-crown, 2s 6d (12½p).

half-bull white (n.)

a counterfeit half-crown.