Green’s Dictionary of Slang

1. a habitual adulteress, although not a professional prostitute.

1889		Barrère & Leland Dict. Sl., Jargon & Cant I 384/1: Sooner than be out of fashion they will tolerate what should be most galling and shaming to them--the thought that by these they are put down among the free lances.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US und.) a prostitute operating without a pimp.

1938

J. O’Hara Hope of Heaven 98: I immediately recognized the girl with him, and so would have at least half the men in the room. She was one of the girls; a free-lance.