Serious, a. — Webster enters it under “serious”
Etymology: [L. serius: cf. F. sérieux, LL. seriosus.]
1. Grave in manner or disposition; earnest; thoughtful; solemn; not light, gay, or volatile. He is always serious, yet there is about his manner a graceful ease. Macaulay.
2. Really intending what is said; being in earnest; not jesting or deceiving. Beaconsfield.
3. Important; weighty; not trifling; grave. The holy Scriptures bring to our ears the most serious things in the world. Young.
4. Hence, giving rise to apprehension; attended with danger; as, a serious injury.
1 more senses
Syn. -- Grave; solemn; earnest; sedate; important; weighty. See Grave. -- Se"ri*ous*ly, adv. -- Se"ri*ous*ness, n.
Shades of meaning
Fernald lists seriousness only as an antonym, under wit.
He lists it without comment; the entries above explain its neighbors.