keep/break your word
break one's word
not to do what one said one would do; not to keep one's promise. (Compare this with keep one's word.) Don't say you'll visit your grandmother if you can't go. She hates people to break their word. If you break your word, she won't trust you again.
keep one's word
to uphold one's promise; to do as one says. (The opposite of break one's word.) I told her I'd be there to pick her up, and I intend to keep my word. Keeping one's word is necessary in the legal profession.