verb: To severely beat somebody in the same fashion and manner as a monkey or ape i.e. with the sides of the fist and forearms, usually in a downward flailing motion. Usually, but not always, said by someone (typically a male) who has hefty forearms and fists that resemble a monkey.
Also, when the attacker is monkey beating someone he is usually does so with little restraint or care for himself or his victim.
"If you change the channel from the Golden Girls I swear to God I will monkey beat you until you are nothing but a wet stain on this carpet!"