Etymology
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vacuous (adj.)
1640s, "empty" (implied in vacuousness), from Latin vacuus "empty, void, free" (from PIE *wak-, extended form of root *eue- "to leave, abandon, give out"). Figurative sense of "empty of ideas, without intelligent expression" is from 1848. Related: Vacuously.
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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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