Diddy's fighting back against the massive $100 million default judgment leveled against him this week ... claiming he never received a copy of the lawsuit -- and, even if he had, the statute of limitations ran out years ago.

The controversial music mogul filed the emergency motion to set aside the massive judgment Thursday ... saying the plaintiff, Derrick Lee Cardello-Smith, never served him with a lawsuit -- and, he didn't know he had to handle the matter until news outlets started reporting on the judgment.

Worth noting ... Diddy's lawyers aren't just trying to get the case thrown out for failing to serve him properly -- they're denying all wrongdoing wholeheartedly. They say the "facially implausible allegations are completely fabricated."

Even if the allegations were true -- which again, Diddy's lawyers say they aren't -- they say the statute of limitations ran out nearly 17 years ago.

Diddy's team's also slamming the lawsuit as frivolous ... 'cause they say the man who filed it is a convicted rapist who is lying about all of the allegations in the lawsuit. They also claim he has a history of filing lawsuits that courts have labeled as frivolous.

ICYMI ... Cardello-Smith was awarded $100 million from a judge after Diddy failed to respond to the complaint Cardello-Smith filed in June.

Cardello-Smith claims back in 1997 he and Diddy were having intercourse with a group of women when he felt Diddy's hand touch his butt. He claims Diddy later spiked his drink and -- when he awoke -- Diddy implied he'd sexually assaulted Cardello-Smith.

The judge set up a payment schedule -- $10 million a month beginning October 1.