1 thoughtful, earnest, sober, sedate, responsible, not reckless or given to trifling (has a serious air; a serious young person).
2 important, demanding consideration (this is a serious matter).
3 not slight or negligible (a serious injury; a serious offence).
4 sincere, in earnest, not ironical or joking (are you serious?).
5 (of music and literature) not merely for amusement (opp. LIGHT(2) 5a).
6 not perfunctory (serious thought).
7 not to be trifled with (a serious opponent).
8 concerned with religion or ethics (serious subjects).
Derivatives:
seriousness n.
Etymology: ME f. OF serieux or LL seriosus f. L serius
Useful english dictionary. 2012.