Green’s Dictionary of Slang

a drink, consisting of the dregs collected from the overflow from the pouring taps, the ends of spirit bottles and similar leavings, which was sold cheap in gin shops, esp. to women.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1870		Hotten Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1894		Brewer Dict. Phrase and Fable I 35/1: alls, tap-droppings. The refuse of all sorts of spirits drained from the glasses or spilt in drawing. The mixture is sold in gin-houses at a cheap rate.
1895		C. Crowe Aus. Sl. Dict 4: Alls, refuse drink, as tap droppings.
1900		Eve. Teleg. (Dundee) 1 Sept. 3/6: The language of the London East-end pub [...] ‘Alls’ — droppings of all kinds of spirits.
1902		H. Baumann Londinismen (2nd edn).
1909		J. Ware Passing Eng. of the Victorian Era 6/2: Alls (Public-House). Waste pot at public-houses. On all public-house pewter counters may be seen holes, down which go spillings of everything. Popular mistrust runs to the belief that these collections are used up – hence the comment upon bad beer. ‘This must be alls.’.