m.p., n. — Green’s Dictionary of Slang

m.p. n.

[abbr. member of the police]

(US/Aus.) a police officer.

1855		‘Q.K. Philander Doesticks’ Doesticks Letters 291: Shall sue the city, for I know that in the fighting business I did my duty as an M. P. according to police usage from time immemorial.
1864, 1867, 1870		Hotten Sl. Dict. 182: M.P. member of the police, one of the Slang titles of the force.
1870		Appleton’s Journal (N.Y.) 30 Apr. 497/1: It’s ’most as good as steam-gratin’, and there ain’t no M.P.’s to poke neither!
1895		C. Crowe Aus. Sl. Dict. 50: M.P., member of the police.
1926		Derby Dly Teleg. 7 Apr. 3/3: Among other slang terms for police-men are rozzers, cossacks, frog; raw lobster, M.P. (member of police) nam.
1960		‘Nino Culotta’ Cop This Lot 40: An American voice from one of the tables called out, ‘Send for the M.P.’s. Send for the M.P.’s.’ Dennis said, ‘Keep the cops out of it or I’ll drop yer.’.