sullen [LME] To be sullen originally related to the idea of being on your own. The source is Old French sulein, from sol ‘*sole, alone’, which came ultimately from Latin solus ‘alone’. Early meanings were ‘single’, ‘solitary’, and ‘unusual’. There is obviously a link between being solitary and being averse to company or unsociable, and such associations led to a shift in sense as the word came to describe someone who is silently gloomy, resentful, and moody.