Green’s Dictionary of Slang

(US Und.) a young woman who occas. works as a prostitute but is not fully committed to the life.

1963		T.I. Rubin Sweet Daddy 67: He got me a chick two years ago, semi-pro kind of [...] mostly making it in stags and French stuff.
1986		J. Ellroy Suicide Hill 226: ‘[Y]ou get complimentary coke within reason, high-class whores, a shot at a few semi-pro ladies’.