Q From Geoff Bird: In one of the Monty Python movies, as a woman falsely accused of being a witch is being carted off to her destiny she says under her breath, that’s a fair cop! Is this the common British slang for being arrested?
A It’s a well-understood British expression, though it has been used so often in second-rate detective stories and police television series down the decades that it has long since ceased to be possible to use it seriously (the Monty Python team was playing on its clichéd status).
It comes from the same root as the term cop for a policeman. This may be from the slang verb cop, meaning to seize, originally a dialect term of northern England that by the beginning of the nineteenth century was known throughout the country. This can be followed back through French caper to Latin capere, to seize or take, from which we also get our capture. (See also the piece on cop, a policeman.) So a cop in this sense was an example of a seizure or capture.
It’s a fair cop was what the essentially good-natured thief with a typically British sense of fair play was supposed to say as his collar was fingered by the fuzz, meaning that the arrest was reasonable and that he really had done what he was accused of doing. You will understand that this is, and always has been, an entirely fictitious view of the relationship between British criminals and the police.