/ ˈsɒləm; NAmE ˈsɑːləm/ adjective
1.
( of a person ) not happy or smiling :
Her face grew solemn.
a solemn expression
SYN serious
OPP cheerful
2.
done, said, etc. in a very serious and sincere way :
a solemn oath / undertaking / vow, etc.
3.
( of a religious ceremony or formal occasion ) performed in a serious way :
a solemn ritual
► sol·emn·ly adverb :
He nodded solemnly.
She solemnly promised not to say a word to anyone about it.
The choir walked solemnly past.
••
WORD ORIGIN
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.