means A visible sign or impression on a surface; a target or goal; a sign of quality or identity; a grade or score; or, in slang, the intended victim of a con or swindle.

from From Old English 'mearc,' meaning a boundary, sign, or limit — a cousin of Old Norse 'mǫrk' and German 'Mark' (border region, hence place-names like Denmark, 'the march of the Danes'). The root sense is a line drawn to divide or designate, which fans out naturally into scratches, targets, and signs. The con-artist 'mark' is a later slang twist: the chosen victim, figuratively the spot you aim your scheme at.