Bob Arum Craps All Over Mayweather, McGregor ... Plop, Plop, Plop

Legendary boxing promoter Bob Arum took out his metaphorical newspaper, unbuckled his metaphorical pants ... and took a big steamy metaphorical crap all over the Mayweather, McGregor fight.

"It's not a fight for Christ's sake," Arum told TMZ Sports ... "There's not even a chance McGregor wins."

Arum says he's not shocked at reports saying tickets to the fight haven't sold out yet -- telling us, "There's a limited number of suckers."