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) (idiom)

fig. to maintain one's resolve for revenge; variant of 臥薪嘗膽|卧薪尝胆[wo4 xin1 chang2 dan3]

to lie

xīn

(literary) firewood; fuel

cháng

old variant of 嘗|尝[chang2]

dǎn

gall bladder