The least a human being has to do to exist is to be ejaculated from the tip of a man's penis into a vagina: Hence, a "fuck nut". A fuck nut is a person with the least amount of intelligence it takes to exist as a human being and nothing more. i.e., they are only as smart or intelligent as they were first squirted-out of their father's nut sack.
Customer: Can you fix my phone? It won't come on.
Tech: Did you charge the battery?
Tech: Well there you go, you fuck nut!