伐毛洗髓 fámáoxǐsuǐ (also fāmáoxǐsuǐ) to undergo a complete transformation
to cast off one's old self; to make a thoroughgoing change; originally meaning to scrape away hair and wash the bone marrow, it is now used to describe a spiritual or physical rebirth, as in 脱胎换骨 tuōtāihuàngǔ or 洗心革面 xǐxīngémiàn