(1):

(a.) Hence, giving rise to apprehension; attended with danger; as, a serious injury.

(2):

(a.) Important; weighty; not trifling; grave.

(3):

(a.) Really intending what is said; being in earnest; not jesting or deceiving.

(4):

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