The term 'plain clothes' refers to a type of clothing worn by police officers and other law enforcement personnel when they are not in uniform. The term 'mufti' is a five-letter word which is used to describe this type of clothing. Mufti is derived from the Arabic word for 'civilian dress'. Plain clothes are typically worn by police officers and other law enforcement personnel when they are conducting undercover operations or surveillance. They are also sometimes worn by detectives and other law enforcement personnel when they are off duty.