Green’s Dictionary of Slang

(orig. US) a fool, also attrib.

1914		L.A. Times 5 June 14: After her trip to Virginia Miss Helen Morton was quoted as saying that Chicago men were ‘nit wits’ and that she fled, not to be married, but to associate for a time with real men.
1921		N. Putnam West Broadway 41: ‘Forget them literary nitwits’.
1923		O.O. McIntyre New York Day by Day 22 May [synd. col.] Some nitwit is trying to wear out a motor horn in the next block.
1932		(con. 1919) Dos Passos Nineteen Nineteen in USA (1966) 674: It’s all my fault for running around like a little nitwit.
1936		‘Josephine Tey’ Shilling for Candles 179: A nitwit blond explained how she read new meaning into Shakespeare.
1942		P. Wylie Generation of Vipers 103: Most of the treatises on what is called ‘mob psychology’ are the work of nitwits.
1950		A. Buckeridge Jennings Goes To School 181: You illiterate nitwit.
1958		D. Stivens Scholarly Mouse and other Tales 32: Can’t be much in it if those nitwits can do it.
1961		Beano 28 Jan. n.p.: Grrr! You little nitwit!
1976		P. Theroux Family Arsenal 173: Because I might lose the lot, nitwit.
1984		W. Ammon et al. Working Lives 72: He was treating me like a nitwit.
2000		Guardian G2 27 June 5: To all appearances, the man is a hopelessly confused nitwit.
2006		C. Hiaasen Nature Girl 88: Honey Santana forgot about the tuxedoed nitwit passed out in the Continental.
2010		(con. 1973) C. Stella Johnny Porno 263: My sister [...] ran to the pastor of their parish, nitwit that she was.