martingale n.

1580s, "strap passing between the forelegs of a horse as part of the harness," from French martingale (16c.), of uncertain origin, perhaps from Old Provençal martegalo, fem. of martegal "inhabitant of Martigue," a commune northwest of Marseilles, making the etymological sense "worn in the manner of the people of Martigue;" or perhaps it is from Spanish almartaga, a word for a sort of halter or rein, from Arabic almartak, in which case it might have been influenced in form by the Provençal word. The nautical sense of  "short, perpendicular spar under the bowsprit-end" is by 1794.