The line 夕阳西下,断肠人在天涯 closes Ma Zhiyuan's 《天净沙·秋思》 (Yuan-dynasty sanqu poem) — one of the most quoted images of grief and homesickness in Chinese literature.
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Literary / poetic. In modern conversation use 心碎 (xīnsuì, 'heart-shattered') or 伤心 (shāngxīn, 'sad') instead.