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Douglas Harper's Etymology Dictionary
numbnuts

stupid or ineffectual person, by 1971, U.S. slang, from numb (adj.) + nuts "testicles;" with suggestion of impotence.

Wiktionary
numbnuts

n. (context chiefly US slang derogatory sometimes humorous English) A slow-witted, unresponsive, or inept person (usually male).

Usage examples of "numbnuts".

Austin was now regarded as even more of a numbnuts, to one level of the work force, and a dilettante, to their superiors.