Green’s Dictionary of Slang

(UK prison) the practice of letting a warder suspect one of holding contraband, only to reveal that one is holding something completely innocuous; once suspicions have been allayed, one can then reveal or pass on the actual contraband, e.g. tobacco.

1954

J. Phelan Tramp at Anchor 39: I began to learn the twiddle, the dairy, the cop and pass.