translation of

Well, "unforgiven" can be translated as "inclemens". This is the nominative masculine singular. Latin is an inflected language with 5 declensions, 4 conjugations, 5 cases and many grammar/syntax peculiarities. A Latin adjective must be inflected according to its function in the phrase, since it can change its number, gender and case because it must agree with the noun it refers to.

Therefore it would best to know the context to give you a correct translation.

Unforgiven
Its use would be calling someone Unforgiven.

Re: Unforgiven

I don't know of a direct way to translate it.
I would suggest
Cui venia non data est
(Literally, "To whom forgiveness is not given")

I agree with George. Latin quite frequently uses relative clauses to express what we in English would express with a noun or adjective.

inclemens means unforgiving, not unforgiven.