Green’s Dictionary of Slang

half-an-ounce n.

[the contemporary measurement of silver at five shillings an ounce]

half-a-crown, 2s 6d (12½p).

1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.