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His stupidity led to the accident.It was a moment of stupidity.
The study of stupidity is called monology.
That is the correct spelling of "stupidity" (inanity, foolishness).
"Stupidity and stupid" are nouns. It is not common to turn a noun into a verb - and even then it is not correct practice.
I know of no verb form of 'stupidity'.
Noun = stupidity
Adverb = stupidly
Adjective = stupid