expression, godan verb (-ku)
break a promise; go back on one's word
A somewhat formal or literary expression for breaking a promise; in casual speech, 約束を破る is more common.
彼は約束に背いて、会議に来なかった。
He broke his promise and didn't come to the meeting.
More common and neutral way to say 'break a promise'; 約束に背く is more formal or literary.
Phrase composed of 約束 (promise) + に (particle) + 背く (to go against, to defy).