solemn in WordNet English dictionary

1. characterized by a firm and humorless belief in the validity of your opinions
   "both sides were deeply in earnest, even passionate"; "an entirely sincere and cruel tyrant"; "a film with a solemn social message"
   earnest  sincere  solemn 
   
2. dignified and somber in manner or character and committed to keeping promises
   "a grave God-fearing man"; "a quiet sedate nature"; "as sober as a judge"; "a solemn promise"; "the judge was solemn as he pronounced sentence"
   grave  sedate  sober  solemn