A phrase meaning a casual gathering of friends or family. The expression is often used by a male to a female as an innuendo when one is not actually present. This use appears jokingly before the statement "UD that" to imply that no real definition exists. However, when the curiosity of the recipient gets the better of them, they find that a definition has been created as an elaborate ploy to conduct a joke.
Girl: Hey I haven't seen you in forever!
Guy: Oh yeah, I know. We should totally have a friendly get-together Saturday night at my house, if you know what I mean.
Guy: Yeah, you and Kelly, Josh, Alex, and Tom should come over to watch some movies.