Synonyms for Word of honor:

n.

guaranty, guarantee, plight, covenant, obligation, solemn word, assurance, engagement, warrant, vow, pledge, promise. promise (noun)
solemn word, vow.
solemn declaration (noun)
plight, assurance, guarantee, vow, pledge, promise, solemn word, warrant.
word (noun)
word.
word of honor (noun)
word, parole.

v.

guarantee (verb)
pledge.

Other synonyms:

solemn word, promise. vow. assurance. guarantee, covenant. guaranty, engagement. warrant. Other relevant words:
guaranty, guarantee, promise, covenant, pledge, solemn word, assurance, engagement, word, warrant, plight, vow.