Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
unassailably is an adverb meaning "in an unalterable and unchangeable manner". It carries one recorded sense.
in an unalterable and unchangeable manner
“his views were unchangeably fixed”
Definitions, synonyms and semantic relations for unassailably come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.