A removable or hinged cover for a hollow receptacle or box. An eyelid. Biology A flaplike covering, such as an operculum. A curb, restraint, or limit: approved a new lid on corporate spending. Informal An act of concealment; a cover: told us to keep a lid on the report until the campaign was over. Slang A hat. Slang An ounce of marijuana. To cover with or as if with a lid.