plain clothes

plural noun

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

pl n

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

n.“ordinary dress” (as opposed to military uniform), 1822; of police detectives, it is attested from 1842. Also plainclothes.