wki give this
>Cop or Copper
The term Copper was the original, unshortened word, originally used in Britain to mean "someone who captures". (In British English the term Cop is recorded (Shorter Oxford Dictionary) in the sense of 'To Capture' from 1704, derived from the Latin 'Capere' via the Old French 'Caper').[5] The common myth is that it's a term referring to the police officer's buttons which are made of copper.
Cop derives from a Gaelic word which has the equivalence of saying, protector, leader, or chief. The terms are almost nearly homophonic but have similar meanings.