While many of us associate the slogan 'Just Do It' with swanky trainers and trendy athleisure wear - it's clear the origins of the phrase come from a far more harrowing place than we would have ever thought.

The phrase, first coined by the sportswear brand back in 1988, has since cemented itself as, arguably, one of the most iconic advertising mottos of the century.

However, fans of Nike will be shocked to find out those three special words were inspired by the last words of a prolific murderer.

David Taljat / Alamy Stock Photo

The famed tagline helped build Nike's giant empire all thanks to the titan of the marketing industry, Dan Wieden.

Wieden worked on a whole plethora of campaigns in his decade-spanning career including Old Spice, Coca Cola and the beloved shoe retailer.

His biggest claim to fame, however, was for the then smaller brand based out of Oregon.

The 'Just Do It' advert, featuring an 80-year-old runner named Walt Stack, went on to skyrocket Nike to success and helped shape the company into the booming company it is today.

Speaking of Wieden's genius marketing strategy, Nick DePaula, an NBA feature writer at ESPN said: "Not only was the slogan great, and also approachable and vague enough that anybody could apply it to whatever it was they were trying to aspire to do."

However, it's clear the inspiration behind the slogan was the farthest thing from 'approachable' given that it hails from double-murderer Gary Gilmore who killed two people in Utah back in 1976.

Gilmore brutally robbed and murdered Max Jensen, a gas station employee and then Bennie Bushnell, a motel manager, the following evening according to US Supreme Court records.

Clark County Prosecutor's Office (Portland Police Bureau #35142)

Records show that Gilmore ended up shooting himself in the hand during the ordeal and headed to his cousin's house in need of assistance.

His cousin, however, turned him into the police when she caught light of what he had done.

Gilmore was subsequently sentenced to death that same year and sent to Utah State Prison before he was executed by firing squad.

When asked what his final words were, Gilmore responded: "Let's do it."

Ring a bell?

Explaining the bizarre slogan source, Wieden told filmmaker Doug Pray in his 2009 documentary Art & Copy: "I liked the 'do it' part of it."

Wieden also once revealed: "None of us really paid that much attention. We thought 'Yeah. That'd work'."

And it sure did.

In a separate interview with design website Dezeen, Wieden revealed: "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."

Now that's some outside-the-box thinking.