Green’s Dictionary of Slang

whacker n.3

[whack n.1 (2)]

(Irish) a small glass of brandy.

[Ire](con. 1930s) M. Verdon Shawlies, Echo Boys, the Marsh and the Lanes 35: He took her for a jorum. She started on whackers, but changed to meejums. He was on half ones.