noun
Describes a person whose mood or attitude changes easily and unpredictably. Often used in a critical or negative sense.
彼は機嫌買いで、さっきまで笑っていたのに急に怒り出した。
He is fickle; he was laughing just a moment ago and then suddenly got angry.
機嫌買いな上司に振り
It's exhausting to be at the mercy of a capricious boss.
移り気 refers to changeability in interests or affections, often in romantic contexts. 機嫌買い focuses specifically on unpredictable mood swings.
Compound of 機嫌 (mood, temper) and 買い (buying, purchasing). The literal sense of 'buying someone's mood' may have shifted to describe a person whose mood seems to be 'bought' or swayed easily, hence fickle. The exact historical derivation is uncertain.