Green’s Dictionary of Slang

legitimately.

1934		B. Appel Brain Guy (1937) 8: Tony wasn’t a cow that could be killed legit.
1946		J.E. Dadswell Hey, Sucker 38: Most of the concession boys [...] prefer to ‘work legit,’ meaning they prefer to distribute mechandise.
1953		J. Thompson Savage Night (1991) 8: He was making more money legit [...] than he’d ever made with the booze.
1962		R. Cook Crust on its Uppers 125: The boys who work these poor squares to death have this way of making money out of them legit.
1979		‘Iceberg Slim’ Airtight Willie and Me 203: She’s dead legit, ain’t no need to dump her.
2011		J.J. Connolly Viva La Madness 15: I could sit and be patient, work hard. [...] legit, year after year.