Green’s Dictionary of Slang

quantum n.

a drink.

1772		Bridges Burlesque Homer (3rd edn) 57: Juno and he have had their quantum, / And play no more at rantum-scantum.
1795		R. Newton Samples of Sweethearts and Wives! [cartoon caption] She’s got her quantum, by jingo!
1870		Mercury (Hobart) 23 Apr. 2/5: [from the Stranraer Free Press] [...] a quantum, a modicum, a thimblefu’.
1873		Siliad 99: I, too, O comrade, quantum suff. would cry [F&H].