Synonyms of serious

1. serious (vs. frivolous), earnest, sincere, solemn, grave, sedate, sober, solemn, overserious, real, thoughtful, serious-minded, sobering, solid: usage: concerned with work or important matters rather than play or trivialities; "a serious student of history"; "a serious attempt to learn to ski"; "gave me a serious look"; "a serious young man"; "are you serious or joking?"; "Don't be so serious!"
2. serious, important (vs. unimportant), of import: usage: of great consequence; "marriage is a serious matter"
3. dangerous, grave, grievous, serious, severe, life-threatening, critical (vs. noncritical): usage: causing fear or anxiety by threatening great harm; "a dangerous operation"; "a grave situation"; "a grave illness"; "grievous bodily harm"; "a serious wound"; "a serious turn of events"; "a severe case of pneumonia"; "a life-threatening disease"
4. good, serious, intellectual (vs. nonintellectual): usage: appealing to the mind; "good music"; "a serious book"
5. unplayful (vs. playful), serious, sober: usage: completely lacking in playfulness
6. serious, difficult (vs. easy), hard: usage: requiring effort or concentration; complex and not easy to answer or solve; "raised serious objections to the proposal"; "the plan has a serious flaw"