When a man has a hypothetical boner for another man. i.e. mancrush. This does not mean that the man is actually sexually attracted to men, just that if he were attracted to one, a certain man would be it.
Dude: I'm can't chill, I'm going to the gym with Alex.
Chick: I figured, you've got a total manboner for him.
Dude: Shut up.