the adverb 'vacantly' is formed by adding the old english adverbial suffix '-ly' to the adjective 'vacant'. 'vacant' itself descends from the latin verb 'vacare', meaning 'to be empty, free, or idle', which is derived from the proto-indo-european root 'eue-', meaning 'to leave, abandon, or be empty'. the core semantic thread from 'emptiness' evolved from a physical sense of an unoccupied space to a metaphorical sense of an unoccupied mind or expression. thus, 'vacantly' literally means 'in an empty manner', which concretely describes an unoccupied place but more commonly describes an absent, blank, or unintelligent mental state.

Structure: vacant (empty) + ly (adverb suffix) vacant: from latin 'vacant-', present participle stem of 'vacare' meaning 'to be empty, free, or unoccupied'. functions as the adjective root. ly: from old english '-līce', an adverbial suffix forming adverbs from adjectives. functions as a derivational suffix.