chaser (n.)
c.1300, "horse trained for chasing," agent noun from chase (v.), probably in some cases from Old French chaceor "huntsman, hunter." Meaning "water or mild beverage taken after a strong drink" is 1897, U.S. colloquial. French had chasse (from chasser "to chase") "a drink of liquor taken (or said to be taken) to kill the aftertaste of coffee or tobacco," used in English from c.1800.