脾气

脾气 (píqi) is a noun meaning a person's temperament or temper — the emotional disposition that shows in how they react, especially to frustration. It is the go-to word for describing whether someone is easily angered (脾气大 / 脾气坏) or even-keeled (脾气好). Two things trip up English speakers. First, 脾气 alone is neutral: it means 'temperament,' not necessarily 'bad temper' — you must add 大, 坏, or 好 to give it a value.

But 发脾气 (fā píqi), 'to release temper,' specifically means to lose one's temper or throw a fit. Second, the second syllable 气 is toneless (neutral tone) here, unlike the standalone 气 (qì). 脾气 describes an ingrained disposition, not a passing mood, which would be 心情 (xīnqíng).