serious

adj. 1 thoughtful, earnest. 2 important, demanding consideration. 3 not negligible; dangerous, frightening (serious injury). 4 sincere, in earnest, not frivolous. 5 (of music, literature, etc.) Intellectual in content or appeal; not popular.  seriously adv. Seriousness n. [latin seriosus]

a. Grave in manner or disposition; earnest; thoughtful; solemn; not light, gay, or volatile.