卧薪尝胆 (wò xīn cháng dǎn): 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); fig. to maintain one's resolve for revenge

xīnchángdǎn

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); fig. to maintain one's resolve for revenge

Goujian slept on brushwood and tasted gall to keep his defeat raw until he could avenge it. For enduring on purpose to come back.