to behave in a way that shows that you are sorry for sth that you have done
WEAR, PUT ON, ETC. SACKCLOTH AND ASHES
Meaning of WEAR, PUT ON, ETC. SACKCLOTH AND ASHES in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005