Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
unfastened is an adjective meaning "not closed or secured". It carries 4 recorded senses.
not buttoned
“the wind picked up the hem of her unbuttoned coat”
not tied
Definitions, synonyms and semantic relations for unfastened come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.