The phrase "solemn word" refers to a promise or statement that is given with great seriousness and sincerity. When looking for synonyms for this phrase, you might consider words like "oath," "pledge," "vow," "commitment," or "assurance." These words all convey a similar sense of gravity and sincerity, and are often used in legal or formal contexts. Other possible synonyms for "solemn word" might include "word of honor," "covenant," "bond," or "guarantee." Whatever word you choose, it should convey a sense of deep conviction and commitment, indicating that the speaker is making a promise that they fully intend to keep.