To remove a person or thing from a previously bestowed appointment, entitlement, status, or position.
When Darci wasted part of an iTunes gift card on a crappy download recommendation from Mark, she had to threaten to disappoint him from his official position as "best music recommender friend ever."
Opening a birthday card in hope of recieving a large amount of money, but all you get is a birthday card.
Disappoint-day card = Disappointing birthday card
Boy 1: Dude, did you get the $60 from your grandparents for that new game in your birthday card?
Birthday boy: No, all i got was a disappoint-day card.