Answer: the prefix "un" means "not."
Step-by-step explanation: an affix is an addition to the base form or stem of a word in order to modify its meaning or create a new word. If the addition is placed at the beginning of a word, it is called a prefix, and if it is placed at the end of a word, it is called a suffix. The word "unforgivable" means "unable to be forgiven or excused" (not forgivable) so based on this definition, we can know that the prefix "un" means "not."