Syllable division and stress derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
uninterestingness is a noun meaning "inability to capture or hold one's interest". It carries one recorded sense.
inability to capture or hold one's interest
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 uninterestingness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.