expression
waiting in vain for an unlikely event
A yojijukugo literally meaning 'waiting a hundred years for the Yellow River to clear', used to describe futile waiting for something that will never happen. Equivalent to English idioms like 'when pigs fly'.
彼が約束を守るのを待つのは百年河清だ。
Waiting for him to keep his promise is like waiting for pigs to fly.
From Chinese legend: the Yellow River is said to be so muddy that it clears only once in a thousand years, so waiting a hundred years is futile. The exact historical derivation is uncertain.