plain clothes

WordReference Random House Unabridged Dictionary of American English © 2025

plain′ clothes′, 

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

Also, plain′clothes′. 

• 1815–25

plain′-clothes′, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

plain clothes noun:plural

1. ordinary clothes, as distinguished from uniform, as worn by a police detective on duty
2. (as modifier): a plain-clothes police officer

WordReference Random House Learner's Dictionary of American English © 2025

plain•clothes /ˈpleɪnˈkloʊz, -ˈkloʊðz-/  also ˈplain-ˈclothes, adj. 

1. (of a police officer, esp. a detective) wearing ordinary clothes while on duty:
   a plainclothes detective.

plain•clothes•man, n. [countable] , pl. -men. 
plain•clothes•wo•man, n. [countable] , pl. -wom•en.