adj. 25B6; adjective
a solemn occasion : DIGNIFIED, ceremonious, ceremonial, stately, formal, courtly, majestic; imposing, awe-inspiring, splendid, magnificent, grand.
he looked very solemn : SERIOUS, grave, sober, sombre, unsmiling, stern, grim, dour, humourless; pensive, meditative.
a solemn promise : SINCERE, earnest, honest, genuine, firm, heartfelt, wholehearted, sworn.
frivolous, light-hearted, insincere.