To ejaculate while in the state of being afraid. Usually occurs when someone is about to find you masturbating and you have to rub one out before they catch you.
I had a fearnut because my mom was about to walk in on me while masturbating.
When masturbating, a guy might ejaculate into his own belly button. Thus giving clean-up a very great obstacle. (Mostly for someone with an inny) Once you think you have gotten all of the semen out of your bellybutton, you reach a little farther, only to discover there is much much more “belly nut”.
Dude, I nutted into my belly button last night and it took me forever to get it all out. My belly nut was so far in there!