Green’s Dictionary of Slang

an amateur, part-time prostitute.

1691		R. Ames The Female Fireships: A Satyr Against Whoring 10: [title] Nocturnal Privateers.
1918		Truth (Melbourne) 23 Mar. 4/7: The [railway] station is a great rendezvous for flappers and mashers on the look-out for a frolic, and also for ‘love birds,’ ‘privateers,’ and ‘pirates’.
1919		Truth (Brisbane) 12 Oct. 11/7: [T]he principal source of infection is not the professional prostitute, but the ostensibly respectable girl or woman popularly known as a ‘privateer,’ ‘love bird,’ or ‘half square tart’.
1986–7		Maledicta IX 149: The compilers ought to have looked farther afield and found: [...] privateer (part-time pro).