Definition 1: An above average looking male/female percieved as a 9 or 10, who couples up with a below average looking male/female, percieved as a 4 or a 5.
Definition 2: An analogy to explain how an unattractive person couples up with an attractive person. The unattractive person usually "shops" and "buys" his/her boyfriend/girlfriend at Walmart, but happens to get lucky and hit the Macy's one day sale.
Mark: Wow,,,,how did Fat Amy hook up with Johnny Football?
Kim: I have no idea.
Mark and Kim (in unison): "Macy's One Day Sale."