The adj serious has 6 senses (first 4 from tagged texts)
1. (29) serious -- (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. (10) serious -- (of great consequence; "marriage is a serious matter")
3. (4) dangerous, grave, grievous, serious, severe, life-threatening -- (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. (3) good, serious -- (appealing to the mind; "good music"; "a serious book")
5. unplayful, serious, sober -- (completely lacking in playfulness)
6. serious -- (requiring effort or concentration; complex and not easy to answer or solve; "raised serious objections to the proposal"; "the plan has a serious flaw")