adj. 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). seriousness n.
[ ME f. OF serieux or LL seriosus f. L serius ]