Long story short: Jabroni is the modern form of Jaboney, American slang used to mock someone, somewhat vaguely, since at least 1931. The online etymology dictionary defines a jaboney as a “naive person, immigrant, hoodlum.”
The Long Story
I started my search with the Goog of course, Goog Books to be exact. I was looking for a reference prior to 1990, that is, prior to The Rock. Some matches were found, but most of those matches seemed to be the Goog book OCR system incorrectly matching Fabroni.
And then I got lucky and ran into this internet question: Jaboney or Jabroney. The consensus on the form was to use the spelling with an “r”, but I was intrigued enough to try Goog Books again. I searched for Jaboney prior to 1990 and hit the jackpot.
“She got the jaboney to marry her, so I didn’t have to support her too long.”
(There is a second use, but Goog cut it off.)
In addition to the reference in this novel, American Speech vol. 6 from 1931 lists the word but unfortunately there is no scan of the page available. Now I’m off to the Seattle Library to see if I can find it.
There are further references in 1934, 1949, 1953, 1969, 1987, 1989, and more.
So it’s pretty certain that those WWE wrestlers didn’t make this one up.