From its value of half of the crown (five-shilling) coin.
half-crown (plural half-crowns)
- (historical) A pre-decimal coin used in Britain and Ireland, equivalent to 30 pence or two shillings and six pence
- 1859, J.C. Hotten, A Dictionary of Modern Slang, Cant, and Vulgar Words
- Half-a-crown is known as an alderman, half a bull, half a tusheroon, and a madza caroon; whilst a crown piece, or five shillings, may be called either a bull, or a caroon, or a cartwheel, or a coachwheel, or a thick-un, or a tusheroon.