Get someone’s goat

To get someone’s goat is to make them annoyed or angry. The origin remains unknown but it is undisputedly American from the early 20th century. There is an unsubstantiated story that it derives from horseracing where a goat was put in with racehorses to calm them. Subsequently, if the goat was stolen, the racehorses would get agitated and jumpy, as would the owners because somebody had got their goat. This story smacks so strongly of folk etymology that it should be treated with circumspection, but because the expression is originally American and this story comes from American sources, it remains the only plausible, but unproven etymology.