卧薪尝胆
[词典] sleep on the brushwood [sticks] and taste the gall -- to nurse vengeance; endure hardships to accomplish some ambition; eat [swallow] the leek; (For days) I have lain, as it were, upon thorns, and my mouth filled with gall.;
[例句]他卧薪尝胆，图谋报复。
He endured hardships to plan retaliation.