Green’s Dictionary of Slang

squiff v.¹

to drink.

1933

‘William Juniper’ True Drunkard’s Delight 227: Perhaps the invitation was to [...] squiff.

In derivatives

squiffed (adj.)

drunk.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1913		J. London Valley of the Moon (1914) 117: ‘Why, Bert! – you’re squiffed!’ Mary cried reproachfully.
1924		O.O. McIntyre Warming a Lobby Chair! 14 Dec. [synd. col.] A chauffeur and a friendly passer by assist a slightly squiffed gent through the revolving door.
1930		Pittsburgh Post-Gaz. (PA) 22 Mar. 17/1: You take the case of one former boxer who when he gets squiffed on home brew or Third Rail varnish is not allowed to talk like the ordinary fellow who gets squiffed because if he does the boys tap their domes [...] and recall the day that Charley White tapped him on the koko.
1931		A. Hardin ‘Volstead Eng.’ in AS VII:2 88: Terms referring to the state of intoxication: [...] Verbs: Squiffed.
1983		S. King Christine 370: He found it unaccountably hard to accept the grapevine version [...] that Buddy and his friends had gotten pretty well squiffed.

In phrases

squiff out (v.)

to collapse through drunkenness.

1953

R. Chandler Long Good-Bye 8: ‘How come I’m here?’ he asked, looking around. ‘You squiffed out at The Dancer’s in a Rolls.’.