solemn.

From Middle English solempne, solemne (“performed with religious ceremony or reverence; devoted to religious observances, sacred; ceremonious, formal; of a vow: made under a religious sanction, binding; religious celebration, celebration of a feast day; famous, well-known; important; grand, imposing; awe-inspiring, impressive; grave, serious; dignified; enunciated or held formally”) [and other forms], from Old French solempne, solemne (“serious, solemn”) [and other forms], or from its etymon Late Latin sōlempnis, sōlennis, from Latin sōlemnis, from sollemnis (“appointed, established, fixed; common, customary, ordinary, ritual, traditional, usual; ceremonial, religious, solemn; festive; annual, yearly”) [and other forms]. The further etymology is uncertain; sollus (“entire, whole”) (ultimately from Proto-Indo-European *solh₂- (“whole”)) + epulum (“banquet, feast”) (in the sense of a ritual; perhaps ultimately from Proto-Indo-European *h₁ed- (“to eat”)) has been suggested.