the word stygian originates from the latin adjective stygius, which was borrowed from the greek stugios, meaning 'of the styx'. the styx was the principal river of the underworld in greek mythology, a dark and dreadful boundary between the worlds of the living and the dead. the waters of the styx were considered sacred and binding, leading to the association of the word with solemn oaths. over time, the primary descriptive quality of the styx—its profound and impenetrable darkness—became the dominant sense of the english adjective, used to describe anything of an exceptionally gloomy, dark, or hellish character.

Structure: styg (river styx) + ian (adjective suffix) styg (from greek stux, the name of the mythological river styx); ian (a latin adjectival suffix meaning 'belonging to' or 'relating to').