IPA: /ˌʌn.fəˈɡɪv.ə.bli/
KK: /ʌnˈfɔrɪɡəblɪ/
In a way that cannot be forgiven or excused; showing a lack of ability to forgive.
He acted unforgivably by betraying his best friend.
Unforgivably → It is formed from "un-" (meaning not), "forgive" (from Old English "forgiefan", meaning to give up resentment), and "-ably" (meaning in a manner that can be). The word "unforgivably" means in a manner that cannot be forgiven or not able to be excused.
Think of 'not' ('un-') being combined with 'giving up resentment' ('forgive') — this helps you remember that 'unforgivably' means something that cannot be excused.