This slang word for a policeman most likely comes from caper, an Old French word meaning to seize, to take. It was adopted into English as to cap by 1589 when it appears in Richard Harvey’s Plaine Perceuall the Peace-maker of England:
Cap him sirra, if he pay it not.
By the beginning of the 18th century, the pronunciation and corresponding spelling had broadened to cop. From Edward Ward’s Dissenting Hypocrite (1704):
If the Cruel Stork should come, He’d Tyrannize and Cop up some [Frogs].
The shift to noun and the meaning of policeman (copper, one who cops) occurred about a century and a half later. From the 1846 Sessions Papers:
I have heard the police called coppers before.
The clipping to cop occurred by 1859. From George Matsell’s Rogue’s Lexicon of that year:
Oh! where will be...all the cops and beaks so knowin’ A hundred stretches hence?
There are several false etymologies floating about. One has it as an acronym meaning Constable On Patrol. Not only is the word older than any other known acronym, but the original form of the noun is copper, not cop.
Other stories have the word coming from either copper buttons on their uniforms or from their copper badges. There is no evidence to support this. The path from the Old French verb is rather clear.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2014, by David Wilton