Go back on your word
Meaning: to not do something that you promised you would do, break a promise
Example: First he said he would help me, but then he went back on his word. I never go back on my word (= not do what I said I would do).
Example: First he said he would help me, but then he went back on his word. I never go back on my word (= not do what I said I would do).
