plainclothesman

[ pleyn-klohz-muh n, -man, -klohth z- ]

/ ˈpleɪnˈkloʊz mən, -ˌmæn, -ˈkloʊðz- /

a police officer, especially a detective, who wears ordinary civilian clothes while on duty.

Origin of plainclothesman

First recorded in 1925–30; plain clothes + man¹

The police sergeant and plainclothesman exchanged a quick glance.

Dan Carter and the River Camp |Mildred A. Wirt
He lingered, but the plainclothesman was staring at the innocent, inviolable parcels about to leave the Embassy for distant parts.

The Pirates of Ersatz |Murray Leinster
She smiled in silent gratitude and allowed the plainclothesman to lead her away from that chamber of horror.

Astounding Stories of Super-Science, March 1930 |Various
Were they with some policeman as they came out of the building or in custody of some plainclothesman?

Warren Commission (3 of 26): Hearings Vol. III (of 15) |The President's Commission on the Assassination of President Kennedy

Followed by a plainclothesman, a middle-aged man entered the living room.

Murder at Bridge |Anne Austin