If you mark off a piece or length of something, you make it separate, for example by putting a line on it or around it. He used a rope to mark off the circle
If a particular quality or feature marks someone or something off from other people or things, it is unusual and makes them obviously different. Her clothes, of course, marked her off from a great number of the delegates at the conference The traditionalist influences within the navy marked it off as a rather old-fashioned institution
(verb.) before 12th century. Middle English, from Old English mearcian; akin to Old High German marcOn to mark, determine the boundaries of, Old English mearc boundary.