Why are police officers called "cops"?

While it’s impossible to trace word origins definitively, the modern usage of "cop" seeps to derive from the word’s definition as a verb: "To get hold of; catch, capture," per Merriam-Webster. The noun evolved from the verb—police officers were "copping" criminals, and eventually, officers became "coppers," then simply “cops."

There are other possible answers, but they’re not supported by any real evidence. One such theory: "Cop" is an acronym for "constable on patrol" or "citizen on patrol." Those are appealing ideas, but they’re probably just backronyms—acronyms created after the fact to explain an older idea. We searched Google Scholar for any references to "constable on patrol" from 1700 to 1900, and the only result appeared in 1894, long after the term “copper" was first used to describe police.

Likewise, there’s no evidence that “copper" once referred to copper badges.

The first known text to define "copper" as "police officer" was 1859’s Vocabulum: Or, The Rogue’s Lexicon. Here’s the book’s definition for "copped":

COPPED. Arrested. "The knuck was copped to rights, a skin full of honey was found in his kick’s poke by the copper when he frisked him," the pick-pocket was arrested, and when searched by the officer, a purse was found in his pantaloons pocket full of money.

Those are absolutely wonderful examples. Since this book is a reference text of sorts, we can assume that "copper" was in fairly regular usage by the mid-1850s. There you have it: "Copped" meant "arrested," and "coppers" were the officers that arrested people.