To have one’s brains knocked out is not a modern Americanism for to be badly beaten in a fight. It dates far back to the late sixteenth century and the first British bareknuckle “fistfights,” as boxing matches were called at the time. Since that time there are several recorded instances of fighters being so badly beaten in the ring that their own mothers didn’t recognize them after the fight, but, thankfully, no actual cases of brains being literally knocked out.