The first time I heard this "nickname" was in November 2018, when it was the subject of a question in Macclesfield Quiz League. While there are references to it on the Internet, I can find no evidence that Jack Johnson was known as such in his lifetime.
The earliest use of it that I could find is in a programme fronted by Lennox Lewis and broadcast in 2010 on BBC Radio 5 Live, with this title. (The programme is no longer available on the BBC website.) An article on the website of the World Boxing Association (the other WBA) also uses this as its title, but doesn't mention it in the text. This article is dated 31 March 2013 – the 125th anniversary of Johnson's birth.
There are a few later references on the Internet, and it's given on Wikipedia as the man's nickname. Knowing Wikipedia's propensity for embracing factoids, this does nothing to convince me of its authenticity; however, because it's on Wikipedia it's now "common knowledge", and that makes it fair game for quiz questions.
© Haydn Thompson 2018