When killer Gary Gilmore was about to be executed by a firing squad, he was asked if he had any final words. He said, "Let's do it." When Dan Wieden of the ad agency Wieden & Kennedy was trying to come up with a slogan for his client Nike, he recalled Gilmore's words.
From Better Marketing:
Dan Wieden was reading an account of this story and was struck by the "do it" part of the phrase. He was amazed that a man, in the face of the ultimate uncertainty that is death, could step through the fire with such confidence.
Then Wieden added the "just" to add emphasis to the "do it" part.
Per Wieden's own words:
"I was trying to write something that would tie it up, so it could speak to women who had just started walking to get in shape, to people who were world-class athletes — and it had the same kind of connection with them."