(one's) word (of honor)
One's sincere promise or vow (about or to do something). I will be in that court to stand by your side during the trail—I give you my word of honor. After the president broke his word about lowering taxes for middle-class earners, I vowed never to vote for him again.
someone's word of honor
someone's trustworthy pledge or promise. He gave me his word of honor that he would bring the car back by noon today.
word of honor
A pledge of one's good faith, as in On his word of honor he assured us that he was telling the truth. [Early 1800s]