TMZ Sports talked to the 37-year-old former UFC champ at Churchill Boxing Club -- director Peter Berg's club -- in Santa Monica ... and we asked the MMA star about Costa's nonstop trash talk.

"July 2nd, international fight week. [A Costa fight is] what the UFC offered. I'm ready to sign the contract, you're making excuses. You're bitching about weight, you're bitching about the fight. Look, I'm right here," Rockhold says, adding, "Whatever the f*** you wanna do, come and step up. It should be an easy win, get yourself back in the limelight. Otherwise, you'd be a bitch running your mouth and walk away."

Of course, 30-year-old Costa, the #4 ranked UFC middleweight (though, he may have to move up to light heavyweight) has been trashing Luke, calling him everything from a guy with a fragile chin to "Luke RockOLD"

"If it's that fragile, come test it. Come and step up. I mean, [Costa] has no clout right now, he has nothing to stand on. He needs a win, he needs to get back in the favorite. He ain't in no f***ing contender ship,  f***in' fighting for the title."

"The motherf***er needs a fight, he needs a name and if he wants to f***in' do it, then let's do it. Otherwise, you're scared and you're a bitch and making excuses. I don't make excuses. I just come and fight."

FYI, Luke, who has beaten everyone from Chris Weidman to Michael Bisping, hasn't fought since losing to Jan Błachowicz in July 2019. It's not that Rockhold hasn't wanted to fight ... but he's dealt with several serious injuries.

But, now Luke is back, and ready to rock with Costa ... who has lost his last two fights (Marvin Vettori and Israel Adesanya).

He also weighed in on Khamzat Chimaev's meteoric rise to UFC stardom ... something Luke says he didn't necessarily deserve, 'cause he hasn't beaten anyone with a name.

"Who the f*** knows what Chimaev is capable of until he fights a real fighter?! He hasn't fought anybody either. I mean, he's talented for sure, but if he can withstand Gilbert [Burns] and get off his ass, get off his back, but I mean he's talkin' [205 lbs.], I mean c'mon. They offered you the fight versus me and you didn't take that fight so don't talk about 205 and [185 lbs.]."

Regardless, a fight with Khamzat would be down the line.