the sudden, unexpected boner you get for no apparent reason. can be brought on due to no stimulation of the penis, just no reason.can be abbreviated as NRB.
Fred:so, I was sitting in the class the other day and I asked the teacher if I could go to the bathroom and I got up and had a no reason boner!
Rick: That shit's funny dawg, did the teacher see you?
Fred:Yah!, along with the whole classroom too!, I was embarassed!