The story you may have heardTwo stories are told: Latin Marci panis 'bread of Saint Mark' (Venice's patron), which Pfeifer calls a folk etymology, and Arabic origins, either mauṯabān (a supposed name for a coin showing an enthroned Christ, 'very hypothetical' per Pfeifer) or marṭabān 'glazed jar', named after the Burmese port of Martaban. Neither Arabic route explains the sound changes, so the Semitic link is unproven.