Dictionary:

vacuity

  (vă-kyū'ĭ-tē, və-)

1. Total absence of matter; emptiness.
2. An empty space; a vacuum.
3. Total lack of ideas; emptiness of mind.
4. Absence of meaningful occupation; idleness: “the crew, being patient people, much given to slumber and vacuity” (Washington Irving).
5. The quality or fact of being devoid of something specified: a vacuity of taste; a vacuity of emotions.
6. Something, especially a remark, that is pointless or inane: a conversation full of vacuities.

[Middle English vacuite, from Old French, from Latin vacuitās, from vacuus, empty. See vacuum.]

noun

1. Total absence of matter: emptiness, vacancy, vacuum, void. See full/empty/capacity.
2. Empty, unfilled space: barrenness, emptiness, nothingness, vacancy, vacuum, void. See full/empty/capacity.
3. A space in an otherwise solid mass: cavity, hole, hollow, pocket, void. See convex/concave.
4. Total lack of ideas, meaning, or substance: barrenness, blankness, emptiness, hollowness, inanity, vacancy, vacuousness. See full/empty/capacity.

Definition: empty-headedness
Antonyms: depth, intelligence

The noun has 4 meanings:

Meaning #1: the absence of matter
  Synonym: vacuum

Meaning #2: total absence of matter
  Synonym: vacuousness

Meaning #3: a region empty of matter
  Synonym: vacuum

Meaning #4: total lack of meaning or ideas
  Synonyms: inanity, senselessness, mindlessness, pointlessness