sincerely UK /sɪnˈsɪəli/US /sɪnˈsɪɹli/ adverb2 •In a sincere or earnest manner; honestly.I sincerely hope they make it home safely. •(US) A conventional formula for ending a letter, used when the salutation addresses the person for whom the letter is intended by his or her name.Please consider the proposal carefully and let me know what you decide. Sincerely, Fred formsmore sincerely · most sincerely