go back on (one's) word
To not do something one said one would do; to renege on something. People are beginning to worry that the president is going to go back on his word and not lower taxes for lower-income families after all. You made a commitment to support our project, and now you've gone back on your word!
go back on one's word
and go back on one's promise; go back on one's pledgeto break a promise that one has made. I hate to go back on my word, but I won't pay you $100 afterall. Going back on your promise makes you a liar.
go ˌback on your ˈword
not do what you have promised; break a promise: He said he wouldn’t charge more than €150, but he went back on his word and gave me an invoice for €200.