Green’s Dictionary of Slang

1. (Aus.) a man who wanders around looking for a casual pick-up; thus on the pirate, looking for a casual pick-up.

1905		Bulletin (Sydney) 21 Sept. 40/1: Rows! Now I’ll tell yer wot causes them. P’r’aps a pirate’ll bring some other pea’s lot ter the room ’n’ label her for every darnce on the bill. ’Er failin’ gets ter ’ear about it ’n’ turns up late, full o’ beer ’n’ bad ideas, ’n’ demands a settlement.
1914		W. Denison Help Wanted 46: I believe there are a lot of pretty little pirates running around from one job to another just hoping some man will try to hold their hand or something.
1918		Truth (Melbourne) 23 Mar. 4/7: By ‘pirates’ he meant people who went to the railway station for no other purpose than to meet each other.
1941		Baker Popular Dict. Aus. Sl.

2. (US) a promiscuous woman, a prostitute.

1931

(con. 1918) E.W. Springs Rise and Fall of Carol Banks 158: In fifteen minutes a Folies Begère pirate would be crying on your shoulder [...] Meanwhile she’d go through your pockets.

3. (US Und.) a pimp who steals a prostitute from a fellow-pimp.

1949

Monteleone Criminal Sl. (rev. edn).

4. (W.I.) an oppressor.

1995

Francis-Jackson Official Dancehall Dict. 39: Pirate 1. oppressors.