An erection that occurs when a man becomes tired, but is fighting hard to stay awake. Instinct boners often occur during boring lectures or when driving late at night and will not go away as long as the urge to sleep persists.
Origin: In order to protect their food supplies and sleeping families from scavengers, early humans evolved the process of instinct boners by frequently masturbating to stay alert while on cave watching duty.
I could not stay awake in class today. I had a huge instinct boner and I spent most of the lecture trying to sneakily tuck it under my belt.