怪物猎人x农场矿石:为什么警察会用cop一词？

这个问题很有意思，查了一下GOOGLE，发现以上两位的都不怎么准确。

其实cop和police或“原指警察制服上的铜扣子，慢慢人们就以扣子代警察了”都没什么关系，但是cop可以说是从copper而来的。

根据下面的英文，我们可以看到cop是1700左右进入英语词汇（原本属于法语中的caper及后来的拉丁语capere，其实是今天capture的意思），是“抓捕、抓获”的意思。1846年，有报刊在cop后面加上了“er”的后缀，表示“抓捕或逮捕犯罪的人”。在1859年，cop作为copper的缩写开始流行起来。

我把英文原文引下来，大家可以做个参考：
Theories for the origin of cop abound, including the copper badge explanation you mention. We'd always been under the impression that the term was an acronym for "constable on patrol." Well, it seems we're both mistaken, as we learned from our roundabout quest for the answer.

Around the year 1700, the slang verb cop entered English usage, meaning "to get ahold of, catch, capture." By 1844, cop showed up in print, and soon thereafter the -er suffix was added, and a policeman became a copper, one who cops or catches and arrests criminals. Copper first appeared in print in 1846, the use of cop as a short form copper occured in 1859.

另一则引文：
[Q] From Billyboy Mackey: “Could you possibly tell me the origin of the slang term for policemen, cops?”

[A] Half a dozen explanations at least have been put forward for this one, including an acronym from “constable on patrol”, which is reminiscent of the story behind posh and quite certainly just as spurious. It is also said to come from the copper badges carried by New York City’s first police sergeants (patrolmen were alleged to have had brass ones and senior officers silver); it is almost as often said to refer to the supposedly copper buttons of the first London police force of the 1820s. Both these stories seem about equally unlikely.
The most probable explanation is that it comes from the slang verb cop, meaning “to seize”, originally a dialect term of northern England which by the beginning of the nineteenth century was known throughout the country. This can be followed back through the French caper to the Latin capere, “to seize, take”, from which we also get our capture.

The situation is complicated because there are—or have been—a number of other slang meanings for cop, including “to give somebody a blow”, and the phrase cop out, as an escape or retreat. Both of these may come from the Latin capere. But it’s suggested that another sense of cop, “to steal”, could come from the Dutch kapen, “to take or steal”. There’s also “to beware, take care”, an Anglo-Indian term from the Portuguese coprador, and phrases like “you’ll cop it!” (“you’ll be punished, you’ll get into trouble”), which could come from the idea of seizing or catching, but may be a variant of catch.

参考资料：
http://ask.yahoo.com/20000315.html
http://www.worldwidewords.org/qa/qa-cop2.htm

学英语的人都知道police是警察的意思，可在美国人们说警察普遍用cop这个词。对英文有些研究的人会说，cop是police的俚语，不是警察的正式用语。那可真是太学究了，因为现在连美国总统、美国国会说警察也常常用cop这个词。

COP这个词是从COPPER变来的，原指警察制服上的铜扣子，慢慢人们就以扣子代警察了

我同意上一位的

习惯口语而已