the word 'sullenness' describes a state of gloomy ill-humor. its core, 'sullen', entered middle english from anglo-french with a sense of being solitary or unique, a meaning which evolved to imply a withdrawn, morose disposition stemming from that isolation. the suffix '-ness', of old english/germanic origin, is a prolific and productive element used to form abstract nouns indicating a state or quality. thus, the compound literally means "the state or quality of being sullen," where the original connotation of lonely aloofness transformed into the modern sense of silent resentment and gloom.

Structure: sullen + ness sullen (from middle english solein, meaning "solitary, sullen," from anglo-french, of uncertain origin, possibly from latin solus meaning "alone") + ness (from old english -nes, -ness, a noun-forming suffix denoting state, quality, or condition).