Nitwit

google

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early 20th century: apparently from nit1 + wit1.

wiktionary

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First attested in the 1910s in the US Northeast. Likely from German nit, dialectal form of nichts(“nothing”), or Yiddish ניט‎ (nit), dialectal form of נישט‎ (nisht, “no”), although some dictionaries give the alternative etymology nit(“louse egg; something very small”) +‎ wit.

etymonline

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nitwit (n.)

also nit-wit, "stupid person," by 1914, American English slang, probably from nit "nothing," from dialectal German or Yiddish, from Middle Low German (see nix (n.)) + wit (n.). Related: Nitwitted; nitwittery.