Green’s Dictionary of Slang

snacking, eating or drinking other than at mealtimes.

1725		New Canting Dict. n.p.: tiffing [...] Drinking.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.