When you were worth something more to someone once; but you managed to find a way to make them change their mind. Used correctly, possibly one of the most painful words in the world. Suggests giving up, no longer caring. Can be used to infer "we can never go back."
I did love you. But now, I don't know. You never did what you said you were going to do. I waited for you, but.. you know, it's just.. disappointing, I guess. That's all.
Ladyfriend: "Come on over, I'm preparing a beer bath"
Mr Winstanley: "You're going to bathe in beer?"
Ladyfriend: "My bratwursts are"
Mr Winstanley: "That's disappointing"