Medieval alchemists ascribed to the planet Saturn a gloomy and slow character. When people are called saturnine, it means they are like the planet — gloomy, mean, scowling. Not exactly the life of the party.
Saturnine is a word you don't hear often nowadays, though you probably know people with saturnine dispositions. The ultimate saturnine character in literature is Heathcliff — and for clarification's sake, that would be the bitter, brooding, obsessive hero of Wuthering Heights, not the lovably pudgy cat of comic-strip fame.