After his defeat by Wu, King Goujian of Yue returned to his state. “He toiled in body and burned in thought. He set a gall bladder by his seat. Sitting or lying down, he looked up at it, and when he ate or drank, he tasted it.” (Records of the Grand Historian)
The Records say nothing about sleeping on brushwood. The two first appear together as wo xin chang dan, sleeping on brushwood and tasting gall, in a piece by the Song writer Su Shi. The idiom describes driving oneself through hardship to grow strong, without forgetting a humiliation.