Get someone's goat

The origin of this expression is not clear. One possible source is the French "prendre la chevre" (to take the goat), which the French use to refer to taking from a person who has nothing to give. The most likely theory, however, is that the phrase comes from racetrack jargon. In former times, it was customary to put a goat inside a high-strung racehorse's stall, because it supposedly had a calming influence on the horse. If, however, someone stole (got) the horse's goat the night before a race, the horse would clearly become upset and most likely lose the race