Derived from baseball dialogue, specifically the New York Yankees, 'did you?!' is a question to reference the certainty (or lack thereof) a player had that his hit was going to be a home run. However the same term can be applied to any situation in which one stumbles upon good fortune, or in which one has a commanding performance or feat of accomplishment.
Shortened form of 'Did you have sex?' or 'Did you do it?'
Sleeze: I had a dream about you last night.
Hottie: Did you?
Sleeze: Nah. You wouldn't let me.
Source: I saw that joke on an episode Casualty, BBC TV Show