Youth, once gone, never returns — just like the yellow crane in the poem.
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history
From 崔颢 Cui Hao (Tang dynasty) 《黄鹤楼》: 黄鹤一去不复返，白云千载空悠悠 — 'the yellow crane, once gone, has never returned; white clouds drift emptily for a thousand years.' Li Bai reportedly refused to write a poem on the Yellow Crane Tower after reading this one.
usage
Usually paired with 白云千载空悠悠 for full effect. 复 (fù) means 'again,' so 不复返 = 'has not returned again' — a stronger, more literary version of 再也不回来.
