Jack Nicholson -- Dissed By Billionaire NBA Megafan ... 'He's Not A Real Fan'

Jack Nicholson is officially UNDER ATTACK by a billionaire NBA superfan ... who claims the actor has TURNED HIS BACK on the Lakers -- and he's pissed!!!

The rich guy going after Jack is Jimmy Goldstein -- who the NBA says spends more money on tickets than any other person in the world. He's also a Lakers season ticket holder.

So, when Jimmy was leaving Mr. Chow the other day, we had to ask how he felt about Jack's decision to essentially turn his back on the Lake Show this year.

That's when Goldstein tore the Joker a new one.

#CelebrityFeud!!!!