In 'The Simpsons' what is the meaning of Mayor "Diamond" Quimby's nickname?

In this case Joe Diamond turns out to be Mayor Quimby. .... is most likely a nod to Douglas Adams answer to the ultimate question of Life the Universe and Everything.

https://en.wikipedia.org/wiki/Phrases_from_The_Hitchhiker%27s_Guide_to_the_Galaxy#Answer_to_the_Ultimate_Question_of_Life.2C_the_Universe.2C_and_Everything_.2842.29

"Joe Quimby's name partly comes from NW Quimby Street in Portland, Oregon, the hometown of show creator Matt Groening. The rest of the character's name is from Portland mobster and chief of police, "Diamond" Jim Purcell."

http://simpsons.wikia.com/wiki/Joe_Quimby

"Purcell was an associate of Jim Elkins, a Portland mob boss, and helped protect Portland's pinball racket and other organized crime activity in the city. In this regard, Purcell was not unique among public servants in Portland, many of whom did business with the highly-influential Elkins.

The character of Diamond Joe Quimby, mayor of the fictional town of Springfield on the television show The Simpsons, created by Portland native Matt Groening, is loosely based on Purcell."

https://en.wikipedia.org/wiki/Jim_Purcell