From serious + -ness.

• (America) IPA: /ˈsɪɹiəsnəs/
• (RP) IPA: /ˈsɪəɹiəsnəs/

seriousness

1. The state or quality of being serious.

   Synonyms: earnestness, seriosity

   Antonyms: jocularity, levity, unseriousness

   • 1918, W[illiam] B[abington] Maxwell, chapter XVI, in The Mirror and the Lamp, Indianapolis, Ind.: The Bobbs-Merrill Company, →OCLC ↗:

     “ […] She takes the whole thing with desperate seriousness. But the others are all easy and jovial—thinking about the good fare that is soon to be eaten, about the hired fly, about anything.”

   • 1963, Margery Allingham, chapter 18, in The China Governess: A Mystery, London: Chatto & Windus, →OCLC ↗:

     ‘Then the father has a great fight with his terrible conscience,’ said Munday with granite seriousness. ‘Should he make a row with the police […] ? Or should he say nothing about it and condone brutality for fear of appearing in the newspapers?

   • July 18 2012, Scott Tobias, AV Club The Dark Knight Rises[http://www.avclub.com/articles/the-dark-knight-rises-review-batman,82624/]

     Though Bane’s sing-song voice gives his pronouncements a funny lilt, he doesn’t have any of the Joker’s deranged wit, and Nolan isn’t interested in undercutting his seriousness for the sake of a breezier entertainment.

• self-seriousness

• French: sériosité, gravité
• German: Ernsthaftigkeit
• Italian: serietà, gravità
• Portuguese: seriedade, gravidade
• Russian: серьёзность
• Spanish: seriedad, gravedad