solemn

adjective

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
2
dignified and somber in manner or character and committed to keeping promises
a grave God-fearing man
a quiet sedate nature

Also useful for “solemn”:Unscrambler— Rearrange the letters in “solemn”