noun
moody person; someone whose mood changes easily
Describes a person whose feelings or attitude shift frequently, often used in a slightly critical or teasing way.
彼は気分屋だから、急に機嫌が悪くなることがある。
He's a moody person, so he sometimes suddenly gets into a bad mood.
気分屋の上司に振り回されて大変だ。
It's tough being at the mercy of a moody boss.
A phrase meaning 'mood changes easily', describing the trait rather than labeling the person.
Compound of 気分 (mood) and 屋 (person with a certain trait). The exact origin is uncertain, but it follows a common pattern for describing people by their characteristics.