‘忽冷忽热’ literally means 'suddenly cold, suddenly hot' and describes rapid, unpredictable fluctuations—either in physical temperature (e.g., weather or body temperature) or emotional state (e.g., mood swings, inconsistent behavior). The repetition of ‘忽’ emphasizes the abruptness and irregularity of these shifts. Each ‘忽’ functions as an adverb meaning 'suddenly' or 'all at once', while ‘冷’ and ‘热’ are adjectives meaning 'cold' and 'hot', respectively. Together, they form a fixed four-character pattern that conveys instability and lack of consistency.

This phrase is commonly used to describe symptoms of illness (like flu or menopause), erratic weather patterns, or interpersonal dynamics—such as someone who alternates between affection and aloofness. It carries a slightly negative or concerned tone, implying discomfort, unpredictability, or unreliability. It’s rarely used in formal scientific contexts but appears frequently in daily conversation, health advice, and literature describing human behavior or natural phenomena.