solemn vs unserious

solemn

adj
  • Inspiring serious feelings or thoughts; sombrely impressive. 

  • Characterized by or performed with appropriate or great ceremony or formality. 

  • Of or pertaining to religious ceremonies and rites; (generally) religious in nature; sacred. 

  • Deeply serious and sombre; grave. 

unserious

adj
  • Not serious; characterized by a lack of seriousness (of attitude, approach, style etc.). 

How often have the words solemn and unserious occurred in a corpus of books? (source: Google Ngram Viewer )