困惫 is an adjective meaning 'drowsy and exhausted' — a state where physical fatigue and mental sleepiness combine intensely. The first character, 困 (kùn), means 'tired', 'sleepy', or 'trapped', often conveying a sense of weariness that impairs alertness. The second character, 惫 (bèi), means 'utterly exhausted', 'worn out', or 'drained', emphasizing deep physical or mental depletion. Together, 困惫 intensifies both dimensions: it’s more severe than just 困 (sleepy) or just 惫 (exhausted) alone, suggesting someone is too tired to think clearly and too sleepy to stay upright.

This word is commonly used in formal writing, medical contexts, or serious narratives — for example, describing patients after surgery, soldiers after prolonged duty, or students pulling all-nighters. It carries a tone of gravity and vulnerability, rarely used lightly or humorously. While it can modify nouns (e.g., 困惫的神情), it most frequently follows verbs like 感到, 显得, or 看起来 to describe a person’s observable condition.