Bonerbrain Share definition Flag One who is of little intelligence and/or someone who acts in an unpleasant manner, a derogatory term as to compare someone to a penis inside of a brain. I was building a giant horse dick in Minecraft and he blew it up with TNT, what a Bonerbrain! Bonerbrain by Duo2Cuo March 31, 2011 Get the Bonerbrain mug.