Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
preposterously is an adverb meaning "so as to arouse or deserve laughter". It carries one recorded sense.
so as to arouse or deserve laughter
“her income was laughably small, but she managed to live well”
Definitions, synonyms and semantic relations for preposterously come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.