The Question:

Why are policemen called "cops"?

The Answer:

There are two major explanations given for this. The more popular explanation is that it's a shortened form of "copper," referring to the copper buttons or copper badges on police uniforms. The more likely explanation is that it comes from the dialect verb "cop," meaning "seize," which ultimately comes from the Latin capere, meaning "to capture." Police officers capture criminals.

—The Fact Monster