卧薪尝胆

wò xīn cháng dǎnwoxinchangdan

1. (lit.) to sleep on brushwood and taste gall (like King Goujian of Yue 勾踐|勾践[Gou1 jian4], in order to recall one's humiliations) (idiom)
2. (fig.) to maintain one's resolve for revenge

Study it in Yanamino