IPA: /ˌʌnprəˈnaʊnsəbl/
KK: /ʌnprəˈnaʊnsəbl/
Something that is very difficult or impossible to say correctly.
Her last name is so long that it's almost unpronounceable.
Unpronounceable → It is formed from "un-" (meaning not), "pronounce" (from Latin "pronuntiare", meaning to declare or announce), and "-able" (meaning capable of). The word describes something that is not capable of being pronounced.
Think of 'not' ('un-') being able to 'declare' ('pronounce') something — that's why unpronounceable means it cannot be declared or spoken.