Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
vacuousness is a noun meaning "indicative of or marked by mental vacuity and an absence of ideas". It carries one recorded sense.
indicative of or marked by mental vacuity and an absence of ideas
“the vacuousness of her face belied her feelings”
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for vacuousness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.