What do you mean, "Forgive and Forget"?

My friend, not long ago, was whining about some slight that she was still churning over, when I tossed off the oft-used, but ill-considered phrase: "Oh, forgive and forget." She stopped in her tracks, looked me squarely in the eyes and said, "I will never forget, and I have no idea what forgiveness means."

Hmmmm.  I had to think about that. This happened several years ago, and I'm still thinking about it. Of course we don't forget. And what is forgiveness, anyway? I have an idea these days...