Calque-loanword from English 'deadline': 死 (dead) + 线 (line). The standard Chinese term is 截止日期 (jiézhǐ rìqī), but 死线 has spread among students, programmers, and office workers because it captures the dread vibe better.
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Internet/office slang. Avoid in formal documents — write 截止日期 or 期限. For casual chat with colleagues or friends, 死线 is fine and quite expressive.
