To beat: originally confined to beating with the hilt of a sword, the knob being, from its similarity to a small apple, called pomelle; in Spanish it is still called the apple of the sword. As the clenched fist likewise somewhat resembles an apple, perhaps that might occasion the term pommelling to be applied to fisty-cuffs.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Polt * PompNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes