Green’s Dictionary of Slang

nit n.¹

1. a fool.

c.1595		Shakespeare Love’s Labour’s Lost IV i: Ah, heavens, it is a most pathetical nit.
1600		J. Day Blind Beggar of Bednall-Green Act IV: Strowd, y’are a Nit, a Slave, and a Pessant.
1622		Massinger Virgin-Martyr II iii: And so, sweet nit, we crawl from thee.
1902		Ade Forty Modern Fables ‘Nit!’ she replied, stopping short and turning the Mackerel Eye on him.
1922		S. Lewis Babbitt (1974) 230: I guess you think I’m an awfully silly little nit!
1941		G. Kersh They Die with Their Boots Clean 196: Shall I scoff this little nit?
1943		H.A. Smith Life in a Putty Knife Factory (1948) 134: With all the audacity of an autograph nit.
1945		Baker Aus. Lang. 130: Fools of one kind and another have carved a considerable niche for themselves in Australian speech and little explanation is needed for any of the following: lardhead, loop, nit, plat (a clipping from platypus) [etc].
1959		W. Hall Long and the Short and the Tall Act I: The creep. The stupid nit!
1962		Beano 10 Feb. n.p.: You short sighted nit.
1971		J. McNeill Chocolate Frog (1973) 35: shirker: [W]e’ll tell this nit a bedtime story.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 72: His accent was so posh it sounded like a Monty Python parody of the old Etonian nit.
1986		S. King It (1987) 757: He could only sit in the straight wooden chair on the other side of Mr Keene’s desk like a nit.
1999		K. Sampson Powder 441: He’d slept like a bear last night. They were all probably trying to call him. What a nit!

2. (Aus. milit.) a police officer.

1924

(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: nit. A policeman.

SE in slang uses

In compounds

nit-head (n.) [-head sfx (1)]

an idiot, a fool.

1999

Guardian Rev. 23 July 2: McCarthy was a nit-head. He didn’t know what was going on.

nit squeeger (n.) [lit. ‘louse-squeezer’]

a hairdresser.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Nit Squeezer, a Hair dresser.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: Nit Squeeger, i.e. Squeezer. A hair-dresser.
1811		Lex. Balatronicum [as cit. 1788].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1788].