Green’s Dictionary of Slang

a general invitation to have a drink.

1899		A. Binstead Gal’s Gossip 146: He stood at the bar with a big cigar, / We asked him what he would have.
1911		J. Masefield Everlasting Mercy 15: Saul is a wonder and a fly ’un. / What’ll you have, Saul, at the Lion?
1916		J. Lait ‘Charlie the Wolf’ in Beef, Iron and Wine (1917) 61: ‘What’ll you have?’ asked the Wolf, digging into his pocket.
1927		(con. 1918) J.W. Thomason Red Pants 156: Quiet night, Gert: Major, what’ll it be?
1938		G. Greene Brighton Rock (1943) 9: What’ll you have?
1940		F.S. Fitzgerald ‘Pat Hobby’s Secret’ in Pat Hobby Stories (1967) 75: Sit down and wha’ll you have.
1943		‘Henry Green’ Caught (2001) 45: Come on now, what are you ’aving?
1975		J. Roe The Same Old Grind 117: ‘What’ll it be [...] The usual, plus one for the lady?’.
1996		(con. 1946) G. Pelecanos Big Blowdown (1999) 75: ‘I’ll have that drink.’ ‘What’s it gonna be?’.