1. dignified and somber in manner or character and committed to keeping promises.
"A solemn promise."
2. characterized by a firm and humorless belief in the validity of your opinions.
"A film with a solemn social message."
early 14c., from Old French solempne, from Latin sollemnis "formal, ceremonial, traditional", perhaps related to sollus "whole."
How to use "sol·emn"?
Awful should not be used of things which are merely disagreeable or annoying, nor of all that are alarming and terrible, but only of such as bring a solemn awe upon the soul, as in the presence of a superior power: the awful hush before the battle. That which is awful arouses an oppressive, that which is august an admiring reverence; we speak of the august presence of a mighty monarch, the awful presence of death. We speak of an exalted station, a grand mountain, an imposing presence, a majestic cathedral, a noble mien, a solemn litany, a stately march, an august assembly, the awful scene of the Judgment Day.
alarming - frightening because of an awareness of danger.
direful - causing fear or dread or terror. "A career or vengeance so direful that London was shocked."
frightful - provoking horror. "A frightful crime of decapitation."
majestic - majestic in manner or bearing; superior to mundane matters. "His majestic presence."
solemn - dignified and somber in manner or character and committed to keeping promises. "A solemn promise."