不是Cap，你说cap的话，会被误认为你在说cab出租车，cop的来历看下面吧，希望你能看懂。
Cop the noun is almost certainly a shortening of copper, which in turn derives from cop the verb. The London police were called bobbies, after Sir Robert Peel who advocated the creation of the Metropolitan Police Force in 1828. Copper as slang for policeman is first found in print in 1846, according to the Oxford English Dictionary. The most likely explanation is that it comes from the verb "to cop" meaning to seize, capture, or snatch, dating from just over a century earlier (1704).
The derivation of the verb is unclear. Most authorities trace it to the French caper and before that to the Latin capere, to seize, take. Other English words derived from capere include capture. Thus, a copper is one who seizes. An alternative theory is that to cop comes from the Dutch kapen, meaning to take or to steal.
The word "cop" has other meanings as well, all connected to "catch" or "snatch":
* To "cop out" meaning to withdraw or escape, or to evade responsibility
* To "cop it" meaning to be punished or get caught
* To "cop a plea" is to try to catch a lesser punishment by admitting to a lesser crime
* "A fair cop" means to be caught in the act.
As with many words, there are several stories floating around positing various origins, almost certainly false. The notion that cop is an acronym for "Constable On Patrol" is nonsense. Similarly, the word did not arise because police uniforms in New York (or London or wherever) had copper buttons, copper badges, or anything of the sort.
The term cop has had derogatory implications. J. Edgar Hoover, the longtime head of the FBI, disliked being called "top cop."
参考资料：http://www.straightdope.com/columns/read/2209/why-are-the-police-called-cops-pigs-or-the-fuzz