Green’s Dictionary of Slang

illegitimate; criminal; also as n., a criminal.

1913		A. Lunn Harrovians 152: Was he a blooming illegit?
1944		J. Archibald ‘Skip Tracer Bullets’ in Popular Detective June 🌐 So Huff had gone illigit [sic]!
1952		(con. 1910s) ‘Harry Grey’ Hoods (1953) 17: Don’t you know everybody’s a crook. Everybody’s illegit.
1999		Indep. Rev. 13 Aug. 11: The charismatic, strung-out and deeply illegit couple.