What'll it be?

and Name your poison.; What'll you have?; What's yours?

Inf. What do you want to drink?; What do you want?; How can I serve you? (Typically said by a bartender or bar waiter or waitress.) Tom: What'll it be, friend? Bill: I'll just have a ginger ale, if you don't mind. Waitress: What'll you have? Bob: Nothing, thanks.