Beating

From Relativespace

Beating is systematic punching, or hitting with a blunt instrument, many times, with the design or effect of causing much pain. It often causes widespread heavy bruising, and sometimes more serious damage, sometimes permanent; and psychological damage. Frequently, to abet this beating, one or more accomplices restrain the victim, often two accomplices, by an arm each.

In the USA it is often called "beating up on".