Saturn, in Roman mythology, was the god of agriculture and time. He was associated with the planet Saturn and was considered to be a serious, somber, and even melancholic figure.
The adjective "saturnine" thus refers to these characteristics: seriousness, gloominess, and a tendency towards melancholy.
The association with Saturn likely stems from the planet's slow and steady movement across the sky, which was seen as a symbol of time's relentless passage and the inevitability of aging and death.