Vacantness comes from vacant plus -ness; vacant is from Old French vacant, from Latin vacāns, present participle of vacāre ('to be empty'). In current use it is a noun meaning "the state of being empty, unoccupied, or expressionless". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The suffix -ness turns a quality into a noun of state, condition, or degree. The -ing ending supplies a participial or action-noun shape. The surface form is compositional: its joined elements remain visible in the modern word. A central historical form is Latin vacāre. Related English forms include vacant, vacancy, vacate, and vacuum. The word's history is thus visible in its construction: the components remain interpretable even after the whole expression has settled into a conventional English meaning.