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1.
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!
by Duo2Cuo March 15, 2011
 
2.
when one has the inability to speak or function because all of the blood has rushed out of their head to their boner. generally makes the affected person very stupid for the duration of the boner. mainly occurs to men with large schlongs.
"she told me i could touche her boobs and i wigged out, i think i said 'i don't need to.' I NEEDED TO."

"Total boner brain dude, ouch."
by twiceonsundays June 02, 2011