Green’s Dictionary of Slang

1. the British Public.

1889		Music Hall & Theatre Rev. 2 Mar. 40/1: His wonderful show, which so fetched the B.P. last year.
1890		Music Hall & Theatre Rev. 1 Mar. 9/1: And after all [...] who are really the judges of your shows? Why, of course, [...] the great ‘B.P.’.

2. (US) a child prostitute [abbr. baby pro under baby adj.].

1971

Go Ask Alice 160: In fact at twelve she was already a BP* *Baby prostitute.

3. (N.Z. prison) a member of the Black Power prison gang.

1982		G. Newbold Big Huey 223: It wasn’t long before the BPs had more or less taken over the whole visiting area.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 28/2: BP abbr. a Black Power gang-member.