百年不遇

bǎiniánbùyù
only met with once every hundred years (drought, flood etc)

CEDICT

  1. (figurative) once-in-a-hundred-years (event); not seen in a hundred years
  2. once-in-a-hundred-years (event); not seen in a hundred years

Wiktionary

比喻甚为罕见，百年也碰不到。

重編國語辭典

极为罕见

維基詞典

Entry Composition

1 百年 bǎinián
  1. hundred years
  2. century
  3. lifetime
2 不 bù
  1. no; not so
  2. (bound form) not; un-
3 遇 yù
  1. (bound form) to encounter; to happen upon; to meet unplanned
  2. (bound form) to treat; to receive