A person or thing one particularly dislikes, dreads, or cannot escape; a persistent bane. A bête noire is not a passing annoyance but a fixed object of aversion, the thing that reliably darkens one's mood whenever it appears.

From French bête noire, literally "black beast." The phrase entered English in the mid-nineteenth century and kept its French spelling, circumflex and all, as a name for the one dread that will not leave.