Green’s Dictionary of Slang

johnny darby n.

a policeman.

1831		‘Gallery of 140 Comicalities’ Bell’s Life in London 24 June 4/4: I’m an Englishman bred and born, and I sharnt submit to any o’ your Johnny Darnies.
1864, 1867, 1870		Hotten Sl. Dict.
1886		Graphic 30 Jan. 23/1: Constables used to be known as johnny darbies, said to be a corruption of the French gensdarmes, and they are still occasionally called johnnies .