Noun
wild hair
(informal) A fervent, usually sudden, desire to do something surprising or unexpected.
get a wild hair to do something
have a wild hair to do something
I get a wild hair up my nose and I want to go. Broderick Crawford
I got into my very theatrical phase. I wore only black: a big black hat and wild hair and wild black clothes, and I carried a sword stick. I went there still looking like Miss Florida, and I came back looking very different. Delta Burke