Someone who is saturnine is very serious, unhappy, and quiet.
Being saturnine is not a good thing. It means someone is not happy and doesn't talk much.
Examples:
He has a saturnine face.
The saturnine man never smiles.
The saturnine girl sat alone at lunch.
The cat looked saturnine after the dog stole its food.
The teacher's face was so saturnine, it scared the kids.
The saturnine man never laughed or played.