break one's word
Violate or fail to observe a promise or contract one has made. For example, You can trust him implicitly; I've never known him to break his word. [c. a.d. 1000]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.