Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
buttoned-down is an adjective meaning "unimaginatively conventional". It carries one recorded sense.
unimaginatively conventional
“a colorful character in the buttoned-down, dull-grey world of business”
Definitions, synonyms and semantic relations for buttoned-down come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.