put on, wear, etc. sackcloth and ashes

put on, wear, etc. ˌsackcloth and ˈashes

behave in a way that shows that you are sorry for something that you have done: Look, I’ve said I’m sorry! What do you want me to do — put on sackcloth and ashes?This comes from the Bible. People wore sackcloth (= a rough material) and put ash (= the grey powder left after something burns) on their face and hair to show that a person they loved had died or that they were sorry for something they had done.
See also: and, ash, put, sackcloth