Describes a person whose mood or attitude changes easily and unpredictably. Often used in a critical or negative sense.
He is fickle; he was laughing just a moment ago and then suddenly got angry.
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.