get someone’s goat, to

To annoy someone, to make a person lose his or her temper. This term is definitely American in origin, but its precise prove- nance has been lost. H. L. Mencken was told that it came from the practice of putting a goat inside a skittish racehorse’s stall in order to calm it down. Removing the goat shortly before the race would upset the horse and reduce its chances of winning, a ruse supposedly planned by a gambler who had bet on the horse’s losing. This explanation seems more far-fetched than a possible connection of the term with the verb “to goad.” In any event, it came into use about 1900.