IPA: /ˌɪn.səˈmaʊn.tə.bəl/
KK: /ɪnˈsɜrˌmaʊntəbl/
Something that cannot be overcome or dealt with successfully.
The team faced insurmountable challenges during the project.
Insurmountable is formed from "in-" (meaning not) and "surmount" (from Old French *surmonter*, meaning to overcome or rise above) and the suffix "-able" (meaning capable of). The word describes something that cannot be overcome or risen above.
Think of something that is 'not' ('in-') able to be 'overcome' ('surmount') — that's why insurmountable means it cannot be overcome.