There has been much discussion of this. IIRC, the basic rule is to be very leery of any alleged acronym etymology prior to the 20th Century. Snopes examines a few alleged acronyms, although not "cop", at http://www.snopes.com/language/acronyms/acronyms.htm

FWIW, here's Quinion's take on "cop"
http://www.worldwidewords.org/qa/qa-cop2.htm


noho ora mai
http://maxqnzs.com/References.html