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.
adj - not up to expectations; "a disappointing performance from one who had seemed so promising" Generally used by posh blokes with long hair and rippled six-packs. Occasionally prefixed by the word 'extremely' to express a greater level of concern over his hopes, desires, intentions or expectations being dashed.
His retiring foe Shrinks from the wound, and disappoints the blow - Addison
That treble 20 has to go down as extremely disappointing - Cox