Cassidy derives copper, in the sense of a policeman, from the Irish word ceapadh, meaning to capture, to appoint, to think. The experts on language, who have actually done some research on the subject, disagree with him. They discount the various folk etymologies about Constable On Patrol or early policemen wearing big copper helmets or big copper badges or receiving a penny a day as wages (!) and they focus on the slang term cop, which is attested from the 18th century and means to seize or to capture. They trace this word to an obsolete French word caper, which ultimately comes from Latin. This word is almost certainly a cognate of the Irish ceapadh as well as capture and captive in English. But there seems no good reason to assume an Irish origin for a term which is found in England and which is hardly found at all in Ireland, where people traditionally talked about peelers and bules (and péas in Irish). Ceapairí are sandwiches in Irish, not policemen!
Here’s an interesting quote from Grant Barrett:
‘It doesn’t require a fluent or native understanding of Irish Gaelic, which I do not have and which Cassidy does not have, either—he is usually careful to leave this point unclear—to see that he’s taking words that have complex meanings and cherry-picking the subsenses that most suit his purposes.’
This is exactly what he’s done in most of his definitions, and it can be seen very plainly here. I personally find it bizarre and a mark of the completely random and ad hoc nature of his associations between Irish and English that he chooses to regard cop and copper as coming from two completely different words. To me, unless there is strong evidence to the contrary, it seems reasonable to assume that cop is a shortened form of copper or that copper is an extension of cop. Cassidy relates copper to the verb ceapadh which means to catch or capture or think or appoint. Cop, says Cassidy, comes from the noun ceap which I am fairly certain is unrelated to the verb ceapadh. The noun ceap is a very complex term with lots of different meanings. Here are the meanings as laid out on WinGléacht (the electronic version of Ó Dónaill’s dictionary):
1. stock, block, base, pad
1a ceap crainn, tree stock; tine chip, log fire; ceap cloiche, stone post;ceap dearnála, darning egg; ceap díslí, diestock; ceap treo, step; ceap na bhfiacla, jaw;
1b ceap magaidh, laughing-stock
3 nave, hub
4 compact body: ceap tithe, block of houses; ceap oifigí, office block
5 (of person) chief; (of person) protector
6 bed, plot: ceap plandaí, plant bed; ceap cabáiste, cabbage bed.
Ceap is certainly not the usual word for chief or boss. People often use saoiste, or bas, or cipín (in Donegal). But I don’t think I’ve ever heard ceap used this way and it isn’t the first thing you’d think of if you heard the word in conversation. And of course, Cassidy doesn’t quote the rest of the definitions. Just as Grant Barrett says, he picks out the meaning that suits him and ignores the rest in order to dishonestly present a case which looks convincing, just like the consummate liar and con-man he was.
To summarise, cop/copper probably come from an English dialect verb meaning to seize or to take, which is ultimately of French origin. This verb is probably a cognate of the Irish word ceap(adh), which is pronounced kyapp(a/oo) but it is very unlikely that ceapadh is the origin of copper.