source: [葬儀に参列する人｜無料GIF画像検索 GIFMAGAZINE 3426934]

formal and dignified:

〈雰囲気･場所などが〉厳粛な, 荘重な (!宗教, 儀式的な荘重さを強調する; → grave) ; 〈儀式などが〉厳かな, 典礼, 伝統にのっとった

e.g. a solemn procession.

not cheerful or smiling; serious:

〈人･態度･表情などが〉まじめな, いかめしい, 真剣な; 〈言葉･宣誓などが〉厳粛な, 心からの; 正式な

e.g. Tim looked very solemn.

characterized by deep sincerity:

e.g. he swore a solemn oath to keep faith.

solemnness noun

Middle English (in the sense ‘associated with religious rites’): from Old French solemne, from Latin sollemnis ‘customary, celebrated at a fixed date’, from sollus ‘entire’.