Green’s Dictionary of Slang

a detective.

1857		‘Ducange Anglicus’ Vulgar Tongue.
1864, 1867, 1870		Hotten Sl. Dict.
1866		Letters by an Odd Boy 161: I can trace something like an idea in a policeman being either a ‘Bobby ‘or a ‘peeler,’ in a detective being a. ‘stop,’.
1882		Dly Dispatch (Richmond, VA) 1 Nov. 3/3: A ‘stop’ is a detective.
1895		Crowe Aus. Sl. Dict. 81: Stop, a detective.
1899		Sketch (London) 22 Feb. 18: ‘I got a charnst to pinch a good haul, but they’s a couple of stops (police) twiggin’ me gyme’.
1901		W.S. Walker In the Blood 143: I ‘fly a kite’ or ‘bill,’ as ‘fence’ I show my skill, / And ‘Johns’ and ‘stops’ I spot most carefullee.