Green’s Dictionary of Slang

(US) a pimp.

1914		Jackson & Hellyer Vocab. Criminal Sl. 58: m’gimp, megimp [...] A pimp; a lover in the vicious meaning.
1926		J.J. Finerty Criminalese 37: Mogimper – One who lives off lude [sic] women.
1928		C. Panzram Journal of Murder in Gaddis & Long (2002) 115: I have met every kind of a crook there is. [...] pimps and McGimps.
1931		‘Und. “Lingo” Brought Up-to-Date’ L.A. Times 8 Nov. K3: McGIMPER: A man who lives on the earnings of a woman of the streets.
1933		Ersine Und. and Prison Sl. 52: magimp, n. A pimp.
1936		Mencken Amer. Lang. (4th edn) n.p.: A pimp is a ... McGimp, fish and shrimp, lover, Latin lover and many others.
1949		W.R. Burnett Asphalt Jungle in Four Novels (1984) 159: One of the dance-hall bums [...] insisted that Louis was using his telephone numbers for business purposes — that he was, in short, a M’Gimp.