Shaquille O'Neal gave an AWESOME (and violent!) pump-up speech to rising UFC star Gillian Robertson ahead of her scrap this weekend ... and yeah, someone tell her opponent to watch out!!!

Shaq's pep talk all went down at his mansion in Atlanta ... when famous MMA coach Din Thomas reintroduced the Big Diesel to 25-year-old Robertson before her tilt with Taila Santos in Vegas on Saturday.

O'Neal was asked to give her some advice -- after all, Robertson is considered by some to be an underdog in the fight and is still trying to make her way up the women's flyweight division -- and the NBA legend sure delivered.

Shaq talked about how he had to take down guys like Michael Jordan, Magic Johnson and David Robinson when he first got into the league ... and then he brought violent karate movies into the speech!

"All the karate movies were the same," Shaq said. "At some point, the student has to kill the master. So, right now, you're the student. You're ranked No. 12. So, you've got 11 masters to kill."

"And then when you get to the top, you've got more work to do. Now, you got to stay at the top."

Shaq then left Gillian with this ... "You got to want it. You got to want it. If you want to be the best, say it. Mean it. And just go for it. And don't hold back."

"If you've got to break some cheeks and break some jaws, do it."