Design Indaba 2015: the advertising executive behind Nike's "Just do it" slogan has told Dezeen how he based one of the world's most recognisable taglines on the words of a convict facing a firing squad (+ interview).

Dan Wieden, co-founder of advertising agency Wieden+Kennedy, described the surprising genesis of the slogan in an interview at the Design Indaba conference in Cape Town last month.

"I was recalling a man in Portland," Wieden told Dezeen, remembering how in 1988 he was struggling to come up with a line that would tie together a number of different TV commercials the fledgling agency had created for the sportswear brand.

"He grew up in Portland, and ran around doing criminal acts in the country, and was in Utah where he murdered a man and a woman, and was sent to jail and put before a firing squad."

Wieden continued: "They asked him if he had any final thoughts and he said: 'Let's do it'. I didn't like 'Let’s do it' so I just changed it to 'Just do it'."

The murderer was Gary Gilmore, who had grown up in Portland, Oregan – the city that is home to both Nike and Wieden+Kennedy. In 1976 Gilmore robbed and murdered two men in Utah and was executed by firing squad the following year (by some accounts Gilmore actually said "Let's do this" just before he was shot).

Nike's first commercial featuring the "Just do it" slogan

Nike co-founder Phil Knight, who was sceptical about the need for advertising, initially rejected the idea. "Phil Knight said, 'We don't need that shit'," Wieden said. "I said 'Just trust me on this one.' So they trusted me and it went big pretty quickly."

The slogan, together with Nike's "Swoosh" logo, helped propel the sportswear brand into a global giant, overtaking then-rival Reebok, and is still in use almost three decades after it was coined.

Campaign magazine described it as "arguably the best tagline of the 20th century," saying it "cut across age and class barriers, linked Nike with success – and made consumers believe they could be successful too just by wearing its products."

The magazine continued: "Like all great taglines, it was both simple and memorable. It also suggested something more than its literal meaning, allowing people to interpret it as they wished and, in doing so, establish a personal connection with the brand."

Born in 1945, Wieden formed Wieden+Kennedy in Portaland with co-founder David Kennedy in 1982. The company now has offices around the world and has "billings in excess of $3 billion," Wieden said.

Wieden revealed in his lecture at Design Indaba that shares in the privately owned agency had recently been put into a trust, making it "impossible" for the firm to be sold.

"I’ve sworn in private and in public that we will never, ever sell the agency," Wieden said. "It just isn’t fair that once sold, a handful of people will walk off with great gobs of money and those left behind will face salary cuts or be fired, and the culture will be destroyed."

He added: "The partners and I got together a couple of years ago and put our shares in a trust, whose only obligation is to never ever, under no circumstances, sell the agency.”

Here is an edited transcript of our interview with Dan Wieden:

Marcus Fairs: You're probably bored to death of this question but tell me how the Nike slogan came about.

Dan Wieden: So, it was the first television campaign we'd done with some money behind, so we actually came up with five different 30 second spots. The night before I got a little concerned because there were five different teams working, so there wasn't an overlying sensibility to them all. Some were funny, some were solemn. So I thought you know, we need a tagline to pull this stuff together, which we didn't really believe in at the time but I just felt it was going to be too fragmented.

So I stayed up that night before and I think I wrote about four or five ideas. I narrowed it down to the last one, which was "Just do it". The reason I did that one was funny because I was recalling a man in Portland.

He grew up in Portland, and ran around doing criminal acts in the country, and was in Utah where he murdered a man and a woman, and was sent to jail and put before a firing squad. And they asked him if he had any final thoughts and he said: "Let's do it".

And for some reason I went: "Now damn. How do you do that? How do you ask for an ultimate challenge that you are probably going to lose, but you call it in?" So I thought, well, I didn't like "Let’s do it" so I just changed it to "Just do it".

I showed it to some of the folks in the agency before we went to present to Nike and they said "We don't need that shit". I went to Nike and [Nike co-founder] Phil Knight said, "We don't need that shit". I said "Just trust me on this one." So they trusted me and it went big pretty quickly.

Marcus Fairs: Most of Dezeen's audience is involved in making products, whether it's trainers or cars or whatever. What is the relationship between what you do and the product?

Dan Wieden: Well if you notice in all the Nike work – I mean there is work that shows individual shoes, but a lot of the work that we do is more talking about the role of sports or athletics. And Nike became strong because it wasn't just trying to peddle products; it was trying to peddle ideas and the mental and physical options you can take. So it was really unusual and it worked very well.

Marcus Fairs: And what about other clients? What do you do if the client just wants you to show the product?

Dan Wieden: Well, it depends on the client as well. But you have to be adding something to a product that is beyond just taste, or fit, or any of that kind of stuff. You have to have a sensibility about the product, a sort of spirit of the product almost.

Marcus Fairs: And do you turn down brands that have product which you don't think is good enough?

Dan Wieden: Oh sure. And we fire clients!