[go] back on {one's} word

to fail to keep a promise or commitment that was previously made

He had promised to help me with the project, but he ended up going back on his word.
When someone goes back on their word, it can damage their credibility and trustworthiness.
It's important not to go back on your word, especially in business deals.
Breaking a promise and going back on your word can damage trust in relationships.
He went back on his word by not showing up at the event as he had promised.