The noun cop, meaning police officer, is sometimes explained, by folk etymology, as being the acronym of constable on patrol (or of chief of police).
Neither is the word cop, or copper, related to copper in the sense of the red-brown metal. As early as 13th June 1864, the Manchester Courier had the following:
The professors of slang, however, having coined the word, associate that with the metal, and as they pass a policeman they will, to annoy him, exhibit a copper coin, which is equivalent to calling the officer copper.
The word copper is first attested in 1846. On 11th May of that year, at the Old Bailey, London’s Central Criminal Court, four men “were indicted for unlawfully assaulting James Brannan and Edmund White, police-constables, in the execution of their duty, with intent to resist the lawful apprehension of a man unknown, who had been found in the act of making counterfeit coin”.
Edmund White, police-sergeant, thus testified:
When I first entered the court a woman screamed very loud, “Jim, Jim, here comes the b— coppers,” and at that moment the money was thrown out — I have heard the police called coppers before.
A copper is literally somebody who cops, that is, who captures, catches, nabs. This is clear from the following definition and example from Vocabulum; Or, The Rogue’s Lexicon, an 1859 “bilingual” dictionary published in New York by George W. Matsell, in which the form cop is first recorded:
Copped. Arrested. “The knuck was copped to rights, a skin full of honey was found in his kick’s poke by the copper when he frisked him,” the pickpocket was arrested, and when searched by the officer, a purse was found in his pantaloons pocket full of money.
George W. Matsell explains in the preface to this book that, although the lexicon he has recorded is that of the New York criminal underworld, it may represent a broader and older language:
Occupying the position of a Special Justice, and Chief of the Police of the great Metropolis of New-York, where thieves and others of a like character from all parts of the world congregate, and realizing the necessity of possessing a positive knowledge of every thing connected with the class of individuals with whom it was my duty to deal, I was naturally led to study their peculiar language, believing that it would enable me to converse with them more at ease, and thus acquire a knowledge of their character, besides obtaining from them information that would assist me in the position I occupied, and consequently be of great service to the public.
It is not, however, to policemen alone that this book will be of service, as these cant words and phrases are being interwoven with our language and many of them are becoming recognized Anglicisms. [...] The vocabulary of the rogue is not of recent date; although it is mainly made up of arbitrary or technical words and phrases, while others are of a purely classical origin. It is a language of great antiquity, and may be dated back to the earliest days of the roving gipsy bands, that infested Europe, from whom the greater portion of it has been derived. It might more properly be termed the Romany or Gipsy language, adapted to the use of modern rogues in all parts of the world, and in which the etymologist will find words drawn from every known language.
And indeed, to take only one example, the definition of hang out in Pierce Egan’s 1823 edition of Classical Dictionary of the Vulgar Tongue (a British book) is strikingly similar to the corresponding definition in Matsell’s Vocabulum:
The traps scavey where we hang out; the officers know where we live. (Pierce Egan)
The place one lives in. “The cops scavey where we hang out,” the officers know where we live. (George W. Matsell)
It is often said that the noun cop, meaning police officer, is a shortening of the noun copper of same sense, because the latter, as we have seen, is first attested in 1846 and the former in the 1859 book by George W. Matsell. But in Vocabulum, both forms are used, which may indicate that they appeared during the same period. For example:
Gunned. Looked at; examined. “The copper gunned me as if he was fly to my mug,” the officer looked at me as if he knew my face.
Fire. Danger. “This place is all on fire; I must pad like a bull or the cops will nail me,” everybody is after me in this place ; I must run like a locomotive or the officers will arrest me.
Lurch. Abandon. “Lurch the booby, he has leaked his insides out to the coppers,” abandon the fool, he has told the officers all he knows.
Hike. Run away. “Hike; the cops have tumbled to us,” run; the officers have seen us.
The verb to cop seems to have originally represented a northern-English pronunciation of the obsolete verb to cap, of same sense. For example, in A Glossary of words used in the County of Northumberland and on the Tyneside (1892), Oliver Heslop wrote:
Cop, to catch, “He copt a butterflee.” “Run after him an’ cop ’im.” Always used in the sense of seizing and correcting. Compare Kep. Copper, a policeman.
Kep, to catch in falling, or to catch and retain at one and the same time.
By the 19th century, the form to cop had become part of the slang of schoolboys, criminals and policemen.
By the turn of the 20th century, the phrasal verb to cop it was used to mean to ‘catch’ it, be punished, get into trouble; also to be killed. For example, in A Glossary of words used in the County of Chester (1885), Robert Holland wrote:
Cop, verb, to catch, both in the sense of capturing, and the semi-slang sense of being scolded:
“I’ve copped it,” said when a boy had been chasing a kitten, and had, at last, got hold of it.
“You’ve copt it.” You’ve caught it, or got a scolding.
During the same period, from the verb, the noun cop came to mean a capture in the phrase it’s a fair cop, an admission that the speaker has been caught doing wrong and deserves punishment. This noun also came to mean an acquisition, a catch, in the phrase no (or not much) cop, meaning of no (or of little) value.