Nike co-founder Phil Knight said he's astounded by the athleisure trend.
"It absolutely blew me away," Knight, who is also chairman of Nike, told CNBC's "Squawk on the Street" on Tuesday. He explained that today's athletic leggings have a purely functional origin in long underwear.
"Forty to 50 years ago, we wore long underwear to run in races because we didn't want to run in sweats and we didn't want to wear shorts because it was too cold," Knight explained.
"Then we dyed the long underwear, and then we made it out of different material and it became muscle tights, and it's now the uniform of the woman walking around New York City," the Nike co-founder said.
Despite Nike's reputation as an athletic brand powerhouse, its origins are more humble. Knight co-founded the athletic apparel giant in 1964 with his track-and-field coach Bill Bowerman at the University of Oregon.