And here I thought Henley was a person's name all these years...
However, please note; the statement in Wikipedia about the name has the classic [citation needed] tag, but further down...
In his biography of Ralph Lauren, the journalist Michael Gross quotes a New York merchant who recalled showing a vintage shirt to a Ralph Lauren buyer: "I showed
this fellow underwear -- a three-button long-sleeve shirt by Johnstown Knitting Mills. He said, 'This is a new shirt.' That's where he got the idea for the Henley shirt."
Michael Gross, Genuine Authentic, The Real Life of Ralph Lauren, pg 22.