Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
unsurmountable is an adjective meaning "not capable of being surmounted or overcome". It carries 2 recorded senses.
not capable of being surmounted or overcome
“insurmountable disadvantages”
incapable of being surmounted or climbed
Definitions, synonyms and semantic relations for unsurmountable come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.