卧薪尝胆 臥薪嘗膽

(lit.) to sleep on brushwood and taste gall (like King Goujian of Yue 勾践 Gōu jiàn, in order to recall one's humiliations) (idiom); (fig.) to maintain one's resolve for revenge; variant of 卧薪尝胆 wò xīn cháng dǎn