From Longman Dictionary of Contemporary Englishcivviesciv‧vies, civies /ˈsɪviz/ noun [plural] informalDC ordinary clothes, rather than military uniform Sam! I didn’t recognise you in civvies.
Examples from the Corpus
civvies• I was going places, thinking and doing things I would never dream of in city civvies.• The early part of the week is the official business bit - clowns in civvies, smoke-filled rooms.• On a Saturday morning there were often more lads down the East End dressed in khaki than in civvies.Origin civvies (1800-1900) civilian 