n. someone who behaves stupidly. (Also a term of address.) : Please stop acting like a nitwit all the time.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
nitwit
1922, from nit "nothing," from Ger. dial. or Yiddish, from M.L.G. (see nix) + wit.