Green’s Dictionary of Slang

late 19C; to punch someone in the face.

1895		Mirror of Life 12 Oct. 11/1: [N]o ‘nobbing’ or ‘capping’ will be allowed, and they also reserve to themselves the right to withhold a prize should the boxers show any signs of ‘banking’ or ‘cutting-up’.
1997		Da Bomb Summer Supplement 3: Cap [...] (v.) 2. To hit someone.