该死 (gāi sǐ) is a mild, colloquial expletive used to express frustration, annoyance, or disappointment—similar to 'damned' or 'blasted' in English. Literally, 该 means 'should' or 'ought to', and 死 means 'death'; together, they form an idiomatic expression implying something 'should die'—but it’s never used literally. Instead, it functions as an interjection, often shouted softly or muttered under one’s breath when things go wrong: a dropped phone, a missed bus, or a typo in an important email.

This phrase is widely understood across Mandarin-speaking regions and carries low social risk—it’s far milder than stronger profanities. It’s commonly used alone ('该死！') or before a noun ('该死的雨！' — 'Blasted rain!'). While not vulgar, it’s still informal and best avoided in formal writing or with elders or superiors. Young adults and peers use it frequently in spoken language, especially in moments of minor misfortune.