- 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
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
Examples from the Web for plain clothes
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
March 22, 2011
“There are half a dozen plain-clothes men in the audience,” he said.The Crimson Tide
Robert W. Chambers
The other two I guessed as plain-clothes men from Vancouver.The Spoilers of the Valley
"Well, yes, sir," admitted the plain-clothes man with reluctance.Steve and the Steam Engine
Sara Ware Bassett
"It's all right, Sam," said the plain-clothes policeman with a grin.The Daffodil Mystery
It was signed by the chief of the Baltimore plain-clothes force.The Winning Clue
James Hay, Jr.
- ordinary clothes, as distinguished from uniform, as worn by a police detective on duty
- (as modifier)a plain-clothes policeman
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for plain clothes
"ordinary dress" (as opposed to military uniform), 1822; of police detectives, it is attested from 1842. Also plainclothes.
Online Etymology Dictionary, © 2010 Douglas Harper