To strike or assault someone violently and severely, typically on the head. He warned me that he was going to brain me if I didn't stop insulting him. The lump on my head is from when a box fell off a shelf and brained me yesterday.
See also: brain
Hit someone hard on the head. For example, The roof collapsed and a bunk of plaster brained him, or I'll brain you if you don't get to those dishes! This term is used both literally (first example) and hyperbolically (second example). [Slang; 1930s]