Solemn
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’.
wiktionary
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], [1] from Old French solempne, solemne(“serious, solemn”)[and other forms], or from its etymonLate 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; [2] 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.
etymonline
solemn (adj.)
mid-14c., "performed with due religious ceremony or reverence, sacred, devoted to religious observances," also, of a vow, etc., "made under religious sanction, binding," from Old French solempne (12c., Modern French solennel) and directly from Latin sollemnis "annual, established, religiously fixed, formal, ceremonial, traditional," perhaps related to sollus "whole" (from PIE root *sol- "whole, well-kept").
"The explanation that Latin sollemnis was formed from sollus whole + annus year is not considered valid" [Barnhart], but some assimilation via folk-etymology is possible. In Middle English also "famous, important; imposing, grand," hence Chaucer's friar, a ful solempne man. Meaning "marked by seriousness or earnestness" is from late 14c.; sense of "fitted to inspire devout reflection" is from c. 1400. Related: Solemnly.
