Green’s Dictionary of Slang

1. (UK Und.) of a crime, successfully achieved.

1865

Leaves from Diary of Celebrated Burglar 51/2: Why, Butty, what about the big ‘stake’ you said was ‘off’? who got it, can you tell?

2. (UK Und.) of an arrest, successful.

1938

F.D. Sharpe Sharpe of the Flying Squad 262: We thought you wouldn’t tumble us, Guv’nor. I suppose it’s off? (meaning that we should pinch them for being in town).