覆 fu4 覆
水 shui3 水
难 nan2 難
收 shou1 收
  1. spilt water is difficult to retrieve (idiom)
  2. it's no use crying over spilt milk
  3. what's done is done and can't be reversed
  4. the damage is done
  5. once divorced, there's no reuniting

Examples

覆水难收。

Info