The adverb 'sulkily' is built upon the core adjective 'sulky', which emerged in the mid-1700s to describe a silent, morose, and resentful mood. While the exact origin of 'sulky' is obscure, it may be linked to an obsolete term meaning 'sluggish', capturing the inert, withdrawn nature of the emotion. The adverbial form was created by appending the prolific Old English suffix '-ly', which transforms descriptive adjectives into modifiers of action. Thus, 'sulkily' literally means "in the manner of one who is sulky," precisely describing the behavior of someone showing silent resentment.

Structure: sulk (to be silently resentful) + i (connective) + ly (adverb suffix) - sulk: Likely a back-formation from the adjective 'sulky' (mid-18th century), of unknown origin, possibly related to obsolete 'sulke' meaning 'sluggish'. - i: A connective vowel used to join the root to the suffix. - ly: An Old English suffix (from Proto-Germanic -liko) used to form adverbs from adjectives, meaning "in a specified manner".