1 Demanding or characterized by careful consideration or application.
1.1 Solemn or thoughtful in character or manner.
1.2 (of music, literature, or other art forms) requiring or meriting deep reflection.
2 Acting or speaking sincerely and in earnest, rather than in a joking or half-hearted manner.
3 Significant or worrying because of possible danger or risk; not slight or negligible.
4 informal attributive Substantial in terms of size, number, or quality.
Late Middle English: from Old French serieux or late Latin seriosus, from Latin serius ‘earnest, serious’.
Being fatuous is my wont.
But today must take a jaunt
Attempting something serious.
Instead of just delirious.
But joy need not be silly.
Bouncing around willy-nilly.
Keeping my face straight
I see the world as great!
[The ODO, dictionary of choice for words of the day, repeated “fatuous”, so it could not be the official entry today. I sneaked it in anyway, of course, never to be too serious.]