The word 'sullen' has a journey from physical solitude to a moody disposition. It originates from the Anglo-French word 'solein', meaning 'alone', which itself came from Old French 'solein', derived from 'sol' (meaning 'sole, alone') from Latin 'solus'. The original sense was 'solitary'. By the late 14th century, the meaning had shifted from describing a person who was physically alone to characterizing one who was gloomy due to being alone or isolated. This semantic evolution from external condition to internal temperament gave us the modern sense of a silent, morose, and ill-humored attitude.

The word is a simplex. It entered English as a whole unit from Anglo-French and Old French, with its core components fused and no longer productively separable in modern English spelling.