殫精極慮 [殚－极虑]

Jyutping daan1 zing1 gik6 leoi6
Pinyin dān jīng jí lǜ

Definitions (CC-CEDICT)
  1. to exhaust one's thoughts and ingenuity (idiom); to think sth through thoroughly
  2. to rack one's brains
  3. to leave no stone unturned