solemn adj.
VERBS be, look, seem, sound She usually had a smile on her face, but now she looked solemn. | become, grow His face grew solemn. | remain
ADV. very He addressed them all in very solemn tones. | rather Her mood was rather solemn.
You can also check other dicts: solemn (English, 中文解释 ), wordnet sense, Collins Definition