(A). Slang for a policeman; so called from Sir Robert Peel, who reconstructed the police system. Bobby, being the nickname of Robert, is applied to the same force. (See Bobby.)
It is an extraordinary circumstance that this word, now applied to a policeman or thief-catcher, was in the sixteenth century applied to robbers. Holinshed, in his Scottish Chronicle
(1570), refers to Patrick Dunbar, who “delivered the countrie of these peelers.” Thomas Mortimer, in his British Plutarch;
Milton, in his Paradise Regained
(book iv.); and Dryden, all use the word “peeler” as a plunderer or robber. The old Border towers were called “peels.” The two words are, of course, quite distinct.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894