A documentary by Doug Pray called “Art and Copy” was released in 2009. It explains the genesis of the offbeat ads from over the years. The film takes some of the most staple slogans known to Americans and examines how they originated and their path to culture glory.
For instance, the Nike slogan “Just Do It” originated from death row. Gary Gilmore, the spree-killer uttered the words “Let’s do it” just before a firing squad executed him in 1977. Dan Wieden of Wieden and Kennedy liked the “do it” part of what Gilmore said and used it for Nike's slogan. The executives consider it a huge success, especially because people started using the slogan in a lot of different contexts, not just sports.
Understandably, Nike doesn't really talk about the origins of the slogan. Not even to employees just starting; but now you know.