Green’s Dictionary of Slang

(US) one who deliberately spills some of their drink so as to give the impression that they are drinking more than is actually the case.

1978

(con. 1963) J. Carroll The Basketball Diaries 27: A ‘spiller’ is the guy who wants to say he had as much as everyone else but knows he’d be floored if he did, so he’s always sneaking over to the window or making fifty trips to the john and spilling half or more of his drink away.