solemnize
To make solemn, or official, through ceremony or legal act.
transitive
The couple chose to solemnize their relationship in a secular ceremony, instead of having a wedding.
To make grave, serious, and reverential.
Wordsworth was solemnized and elevated by this his first look on Yarrow.
September 27, 1873, John Campbell Shairp, "Wordsworth's Three Yarrows", in Every Saturday
Where it comes from
From Middle English solempnisen, from Old French solemnisier, from Medieval Latin solemnizare, from Latin solemnis.
Adapted from the Wiktionary entry solemnize (CC BY-SA 4.0).