Green’s Dictionary of Slang

(US) used as an emphatic no, also added to positive assertions to give a negative meaning e.g., ‘I should say nit!’.

1893		Sun (NY) 21 May 28/1: ‘D’ye tink a Sister’ll let you die widout doin’ all she can fer ye? Nit, nit; I guess nit’.
1896		Ade Artie (1963) 9: I should say nit.
1899		W.J. Kountz Billy Baxter’s Letters 5: I let go with the first barrel, right into the center of the bunch. Nit duck. Then the second barrel went off of its own accord. I’ll swear, Jim, I had nothing whatever to do with it. Anyway, nit duck.
1900		A.H. Lewis ‘Mulberry Mary’ in Sandburrs 10: But Mary says ‘Nit! couple of times nit!’.
1904		Sydney Sportsman (Surry Hills, NSW) 11 May 2/3: Nit, you ‘chain smokers’.
1908		‘O. Henry’ ‘A Lickpenny Lover’ in Voice of the City (1915) 30: Nit; he’s too cheap a guy for that.
1914		E. Dyson Spats’ Fact’ry (1922) 92: ‘What price bein’ bes’ man, Cobber?’ [...] ‘Nit. Not any in Georgie’s, please.’.
1927		AS II:11 475: List of the variants of ‘no’ [...] nope, nit, nitsky, nix, nixy, nah, naw, aber nit [etc.].
1931		(con. 1890s) A.F. Harlow Old Bowery Days 493: Do you want to put me on the bum, like Tim Campbell and Sulzer? Nit!
1942		Life 5 Jan. 57: A fine bunch of statesmen they got in this town — nit.