Civvies are ordinary clothes — jeans, a shirt, a jacket — when you say them against a uniform. Soldiers, sailors, and sometimes police use the word when they take the uniform off and dress like everyone else. People almost always keep it plural, and the natural frame is in civvies: you would not put it on a shop label. It is informal, a little old-fashioned in civilian life, but still alive in military talk, especially British. A civvy (singular) is more often a civilian person than one piece of clothing, and civvy street is forces slang for ordinary life outside the army.