Origin of solemn
In English, the extension of solemn from applying to rites, ceremonies, holy days, or oaths to nonreligious actions or feelings arose in the mid-15th century. One imagines early religious rites and ceremonies as being (like modern ones) serious and reverential affairs, and that the individuals performing or participating in them did so with a corresponding grave and serious demeanor. So it is not hard to see how the current sense of "grave, sober, or mirthless" developed: applying first to the people who participated in religious rites, and then losing the connection with the rites themselves.
Examples from the Web for solemn
They stood in a single row, united by solemn respect as the Liu family remained inside.
Somewhat coyly, Skidmore admits that “Richard was to break this solemn vow in spectacular style.”Three Dicks: Cheney, Nixon, Richard III and the Art of Reputation Rehab|Clive Irving|July 27, 2014|DAILY BEAST
The parade was solemn, with reverent music and the call-and-response singing of two choirs.The First Americans to Observe the 4th Were Moravian Pacifists|Linda C. Brinson|July 4, 2014|DAILY BEAST
No putdowns, no jokes, no frivolity whatever—he was most solemn and his eyes focused somewhere far beyond the back of my head.What It Was Like to Watch the Beatles Become the Beatles—Nik Cohn Remembers|Nik Cohn|February 9, 2014|DAILY BEAST
Once again, we are approaching that most solemn and perplexing American social ritual—the Super Bowl party.Your Super Bowl Etiquette Guide From Food to Clothes to What Not to Say|Kelly Williams Brown|February 1, 2014|DAILY BEAST
And it was not, be it remembered, the work of a capricious and cruel despot; it was the tyranny of a solemn legislative assembly.A Book of the Play|Dutton Cook
From Bossuet to Pascal is to pass from the solemn splendour of the church to the chill of the crypt.Critical Miscellanies (Vol 2 of 3)|John Morley
Meanwhile Jeliaboff had heard the solemn tolling of the bells in his prison cell.The White Terror and The Red|Abraham Cahan
I always thought you a hundred years old, Harry, with your solemn face and grave air.The History of Henry Esmond, Esq.|W. M. Thackeray
Among them flesh and wine were seldom used, except at sacrifices at solemn feasts.Vegetable Diet: As Sanctioned by Medical Men, and by Experience in All Ages|William Andrus Alcott
British Dictionary definitions for solemn
Word Origin for solemn
Word Origin and History for solemn
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" (see safe (adj.)).
"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.