The adverb 'sullenly' derives from the adjective 'sullen', which entered Middle English from Anglo-French with a primary sense of being solitary or alone. This solitary state became metaphorically associated with a gloomy, unsociable, and morose disposition. The adverbial suffix '-ly', of Germanic origin, is then appended to indicate the manner of an action, transforming the descriptive quality into a descriptor for behavior. Thus, to act sullenly is to behave in a manner characterized by the silent, withdrawn, and ill-humored attitude historically linked to being alone.

Structure: sullen + ly - sullen: From Middle English solein (meaning "solitary, sullen"), from Anglo-French solein ("alone"), ultimately from Latin solus ("alone, sole"). - ly: A suffix of Germanic origin (Old English -līc) used to form adverbs from adjectives, meaning "in a specified manner".