黄河清

Traditional (Taiwan): 黃河清

huánghéqīng ㄏㄨㄤˊ ㄏㄜˊ ㄑㄧㄥ

    • something that will never happen
    • impossible hope

Character by character

Saying俗语俗語

Idiom惯用语 · Rarely used now

Literally
the Yellow River runs clear
Meaning
Something that will never happen, since the Yellow River is full of silt. Used of an impossible hope; also a sign of a rare golden age.

Sources