Sullen continues Middle English solein, which could describe someone alone. The emphasis shifted towards an unsociable mood. The same word can also colour a description of a gloomy morning.
sullen
Gloomy, resentful or unwilling to communicate.
Being alone became a mood, then an atmosphere
How sullen reached English
oldest firstLatin
solus
‘alone’
into Anglo-French
Anglo-French
sulein, solain
into Middle English
Middle English
solein
‘solitary’
became English
English
sullen
The story of sullen
Related
The old social condition differs from the modern emotional description.