IPA: /ˌʌn.rɪˈtɜː.nə.bəl/
KK: /ʌnˈrɪtɜrnəbl/
Something that cannot be sent back or returned.
The store has a policy that all sale items are unreturnable.
Unreturnable → It is formed from "un-" (meaning not) and "return" (from Latin "returnare", meaning to come back) and "-able" (meaning capable of). The word "unreturnable" means not capable of being returned.
Think of 'not' ('un-') being applied to 'coming back' ('return') and 'capable of' ('-able'). This helps you remember that unreturnable means something that cannot come back.