I said it at a driver hesitating to turn into the lane I occupied in slow traffic. First I waited and then politely waved him in. He just sat there, looking at me, and then this came out of my mouth, barely audible: "Go, numbnut." Yeah, not nice at all. Grossly impatient. Though it's the type of thing I will say alone in the safety my car. I'm sure we all do. But what the hell is this name I'm calling the other driver? I thought. "Numb nut"? Why not "numb nuts"? "Numbnuts"? "Numb-nut"? "Numnut"? With or without spaces and hyphens–there does not seem to be one standard version or spelling of the term, but I lean towards "numbnut", as a variation of "numbskull". A stupid person. "Nut", like "skull", the slang word referring to the person's head and the lack of processing therein.
But, what about "numb nuts", with an "s"? Have we transferred the focus from the cranium to the testicles with that "s"? I believe we have, and likely more people use "numbnuts" over "numbnut". Common usage seems to come from military drill instructors flexing the term to mean "impotent" as an insult to recruits. It makes sense. No one really uses the word "nut" to mean "head" anymore, but nut certainly evokes the testes reference, so if they add the "s" and they can really insult a man by saying that their testicles are numb, and they are incapable of performing sexually. The Unofficial Unabridged Dictionary for the Marines has it listed as someone who is "ignorant or untrainable."
As far back as the 1800's (and probably before) the term "numb head" was tossed around as a regular insult, and then the slang term of "nut" for "head" entered the equation and we easily connect the dots. But, of course, we found a way to make an intelligence insult about a guy's balls. You can't expect to use the word "nut" and not have numbnuts spin it to a sexually derogatory version–that's what jerky older brothers DO. It's like when you use the word "asinine" (extremely stupid or foolish). It sounds like a swear because we really lay into the "ass" part of the word to make it seem even more severe.
'Tillie Numbnuts' is a name in Blair Treynor's 1946 detective novel She Ate Her Cake, given to a character who won't put out. Most other early uses attribute the term "numbnuts" to the 1960's, but it is easy to see that it was probably already in wide usage in the military during WWII. And, with the military back in those days being exclusively male, the term reflects that. When was the last time you heard a woman call anyone "numbnuts"? I have not but I welcome it.
I will still use the term as a slam on one's intelligence, rather than the testicular variety, especially when I read how in 2012 former New Jersey governor Chris Christie once used it describing an openly-gay state assembleyman who was advocating for gay marriage. Yeah, that's pretty vile and I would prefer to leave people's nuts out of this one altogether.