Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
unimpeachably is an adverb meaning "without question". It carries one recorded sense.
without question
“Fred Winter is unquestionably the jockey to follow”
“they hired unimpeachably first-rate faculty members”
Definitions, synonyms and semantic relations for unimpeachably come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.