Green’s Dictionary of Slang

a glass of liquor, a dram.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: squail a dram.
1809		G. Andrewes Dict. Sl. and Cant.
1821		Flash Dict.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.
1859		Matsell Vocabulum.