1 general:: 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]
American Heritage Idioms
2 general:: Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs