get a wild hair up (one's) ass

1. rude slang To become intensely annoyed or irritated. Everybody who lives on this side of town gets a wild hair up their ass about the traffic delays from construction of the new inter-city tramline.

2. rude slang To become obsessed (with someone or something), often when said obsession is strange or unexpected. Mom got a wild hair up her ass about going to Paris all of a sudden. She doesn't know the first thing about France!