What does Boxing Day have to do with boxing?
early 14c., from Old North French gartier "band just above or below the knee" (Old French jartier, 14c., Modern French jarretière), from garet "bend of the knee," perhaps from Gaulish (cf. Welsh garr "leg"). Garter as the highest order of knighthood (mid-14c.), according to Froissart was established c.1344 by Edward III, though the usual story of how it came about is late (1614) and perhaps apocryphal. The verb is mid-15c., from the noun. Garter snake (U.S.) so called from resemblance to a ribbon. Garter belt first noticed 1913.