plain clothes

clothing other than one's uniform, especially civilian clothes worn on duty by a police officer.

Origin of plain clothes

First recorded in 1815–25

plain-clothes, plain·clothes, adjective

Outside at the taxi stand, the men approached and finally identified themselves: They turned out to be plain-clothes police.

How a Blogger Blocked Sex Slavery |Abigail Pesta |March 22, 2011 |DAILY BEAST
I fooled the plain-clothes chap who was following me round, gave him the slip, and then came to see ye.

Frank Merriwell's Pursuit |Burt L. Standish
Immediately after them came the sergeant, Inspector Badger, one or two plain-clothes men, and finally the divisional surgeon.

The Vanishing Man |R. Austin Freeman
At any rate, none of my plain-clothes men has seen hide nor hair of one of the lot since, either in town or out.

Cleek of Scotland Yard |Thomas W. Hanshew

Then there were policemen everywhere, and every other man nearly was a plain-clothes man or a detective.

Colonel Crockett's Co-operative Christmas |Rupert Hughes
Ruth and Helen had sharp eyes, and they saw two plain-clothes men standing by to watch the forthcoming passengers.

Ruth Fielding Down in Dixie |Alice B. Emerson

plain clothes

ordinary clothes, as distinguished from uniform, as worn by a police detective on duty
(as modifier)a plain-clothes policeman