According to the Ambling Alp Songfacts , the song's title refers to the nickname of a 1930s Italian boxer Primo Carnera, who won the 1933 heavyweight championship and was an opponent of American prizefighter Joe Louis.
Now kid, I know I haven't been a perfect man
And I've avoided doing things I know I can
But if I've learned one thing the tattoo on my arm
Will burn into my thumb it would be that