'vacant' originates from Latin, specifically the word 'vacans' (present participle of 'vacare'), where 'vac-' meant 'empty' or 'to be empty/available'.

'vacant' passed into Old French as 'vacant', then into Middle English as 'vacant', and the adverbial form 'vacantly' was formed in English by adding the suffix '-ly'.

Initially it meant 'empty, unoccupied' (literal sense from Latin), but over time it also developed the figurative meaning 'showing lack of thought or expression', which is common in modern usage.