Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
self-assertiveness is a noun meaning "aggressive self-assurance; given to making bold assertions". It carries one recorded sense.
aggressive self-assurance; given to making bold assertions
Definitions, synonyms and semantic relations for self-assertiveness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.