1. a police officer or detective who wears ordinary civilian clothes rather than a uniform while on duty
Usage: often used in the singular or plural form 'plainclothes officers' or 'plainclothes detectives'; the adjective 'plainclothes' is more commonly used than the noun form

Examples

• The plainclothesman followed the suspect through the crowded market without being noticed.
• Several plainclothes officers were stationed at the airport to monitor suspicious activity.
• She worked as a plainclothesman for the narcotics division for five years.
• The plainclothesman's badge was hidden beneath his jacket.
• Plainclothes detectives often work undercover to gather evidence.
• He was trained as a plainclothesman before being promoted to detective sergeant.