1. To displease a person by not delivering something hoped for: The mail Lynn was wishing for didn't come and this disappointed and saddened her a whole lot!

Jane knew that she would disappoint him when she turned down his invitation to the party.
2. To fail in meeting an expectation or gratifying a hope; to let down: When the players lost the basketball game, they certainly disappointed the coach and some of their parents as well.
3. To thwart or to frustrate: Sally didn't want to disappoint the trust her parents had in her.