Dennis, I think this question didn’t get no respect because many people think things are sometimes what they seem (and often they are!) – kind of like asking the origin of numbskull or dimwit. But I guess there is the outside possibility that it derived from benumbed crazy people or some oblique reference to salted and unsalted. In any case, we should check for when it first came into print. My personal view is that it was coined by the guy who invented the bicycle when he got into a tiff upon returning from his first long ride.
Cassell’s Dictionary of Slang
NUMBNUTS noun [1940s and still in use] (US) an idiot, a fool (cf. DUMBNUTS; NUMBHEAD; NUMBY) Standard English ‘numb’ + NUTS]
NUTS noun [mid 19th century and still in use] the testicles
Dictionary of American Slang
NUMB-NUTS by 1960s 1. noun: A despicable person; =JERK, LIMP-DICK: ‘You gotta get a better job, numb-nuts’—George V. Higgins 2. modifier: ‘. . . not the numbnuts chatter cornballs like bob Hope or Yellowman peddle’—Village Voice
Ken Greenwald (Fort Collins, Colorado – U.S.A.)
Reply from Ken Greenwald (Fort Collins, CO - U.S.A.)