'stygian' originates from Latin, specifically the word 'stygius,' which referred to the River Styx in Greek mythology, a river that formed the boundary between Earth and the Underworld.

'stygius' transformed into the English word 'stygian,' maintaining its association with darkness and the underworld.

Initially, it referred to anything related to the River Styx, but over time it evolved into its current meaning of extreme darkness or gloom.