Etymology
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vacuity (n.)
late 14c., "hollow space," from Latin vacuitas "empty space, emptiness, absence, vacancy, freedom," from vacuus "empty," from PIE *wak-, extended form of root *eue- "to leave, abandon, give out." Originally in anatomy. Meaning "vacancy of mind or thought" is attested from 1590s.
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*eue- 
*euə-, Proto-Indo-European root meaning "to leave, abandon, give out," with derivatives meaning "abandoned, lacking, empty."

It forms all or part of: avoid; devastation; devoid; evacuate; evanescent; vacant; vacate; vacation; vacuity; vacuole; vacuous; vacuum; vain; vanish; vanity; vaunt; void; wane; want; wanton; waste.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit una- "deficient;" Avestan va- "lack," Persian vang "empty, poor;" Armenian unain "empty;" Latin vacare "to be empty," vastus "empty, waste," vanus "empty, void," figuratively "idle, fruitless;" Old English wanian "to lessen," wan "deficient;" Old Norse vanta "to lack."
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