Mardi Gras (n.)

 "Shrove Tuesday, last day of carnival, day of eating and merrymaking before the fasting season of Lent," 1690s, French, literally "fat Tuesday," from mardi "Tuesday" (12c. in Old French, from Latin Martis diem "day of the planet Mars;" see Tuesday) + gras "fat," from Latin crassus, "thick," which is of unknown origin.