Several popular etymologies, all certainly false, exist for this word meaning policeman. One says that it is an acronym standing for Constable On Patrol. Another says that the first policemen in London (or another city--it varies in the telling) had copper buttons on their uniforms. Yet another says that it was not buttons, but a copper badge that gave them the name.
While the ultimate origin is disputed, most authorities agree that it is a shortening of copper. Cop was first used in 1859 and copper predates it from 1846. Copper, as slang for policeman, derives from the verb to cop, which dates from 1704 and means to catch. The OED2 notes that an 1864 newspaper stated that people would exhibit a copper coin as they passed a policeman, in effect calling them copper. This may have been the beginning of the confusion with the metal copper.
The ultimate origin of the verb copper is disputed. It either derives from the Dutch kapen, meaning to take. This in turn comes from the Old Frisian capia, meaning to buy. The other choice is that it derives from the French caper, to take, and ultimately from the Latin capere.
http://www.straightdope.com/columns/read/2209/why-are-the-police-called-cops-pigs-or-the-fuzz
http://orvillejenkins.com/words/cops.html
