Green’s Dictionary of Slang

a policeman.

1819		P. Egan Key to the Picture of the Fancy going to a Fight 28: No Hornies or Pigs ever being present to interrupt their [i.e. pickpockets’] impudent attacks.
1881		Newcastle Courant 2 Sept. 6/5: Sam knew that his game was up, but not caring to assist the horny to a conclusion, he walked with apparent unconcern to the corner.