A stupid cunt with half a brain. For example, maybe used if someone does something stupid or makes a stupid decision
Daniel: "Bro your such a fucking halfnut"
Issac: "Fuck off, your the one in the lowest class you Half-nutted Fuckwitt"
a person who is a bit slow and dim witted, due to the fact that only half of the sperm (aka. "nut") that reached the egg, made it through the wall of the embreo...leaving the fetus (and forthcoming person) lacking in some basic skills and cognitive understanding of anything
Dude : "Does that guy realize he's hitting on his boss's wife or does he just not care?"
Other Dude : "He wouldn't know either way. The poor guy is a half-nut from some inbred trailer-town in some forgotten county"