PLEASE SHARE YOUR THOUGHTS! I seek to understand others. Bible Summary. A simple definition of unforgiveness is “a grudge against someone who has offended you”. Another definition of unforgiveness is not having the compassion to forgive. Unforgiveness is a sin that causes us to think and do evil things. None of us are perfect so why does it seem that there are so many unwilling to say they are sorry and unwilling to forgive.