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 brooding, bitter, obsessed hero of "Wuthering Heights," not the lovably pudgy cat of comic-strip fame.