Wear, put on, etc. sackcloth and ashes
Definition 1 of 1
Pro Tip 1/3Historical Origin
Originates from ancient practices of mourning and penance, symbolizing extreme humility. 
The CEO wore sackcloth and ashes during the press conference to show contrition for the company's mistakes.
Pro Tip 2/3Figurative Use
Used metaphorically in modern times to express deep regret for wrongdoings, not literal dressing. 
The company had to put on sackcloth and ashes after the environmental disaster they caused.
Pro Tip 3/3Public Apology
Often suggests a public or very visible display of remorse, rather than a private apology. 
Following the outage, the service provider donned sackcloth and ashes in front of its users.