That's UFC welterweight champ Kamaru Usman reacting to Conor McGregor after the Irishman suggested the two could fight down the line.

After Kamaru broke Colby Covington's jaw and spirit at UFC 245 on Saturday, Conor tweeted out "145. 155. 170" ... and since he's won belts at 145 and 155, the insinuation is that he wants a crack at Kamaru's title.

So, we asked Kamaru about a possible scrap with McGregor -- and Usman didn't hold back.

"You saw what Khabib did to him," Usman tells TMZ Sports ... "Like, oh my god. It wouldn’t even be fair if I fought Conor."

"Much respect to Conor for what he’s done for this sport and transcended it, but nah, this is not what you want."

"Sit down, little man! Sit down! Learn to walk before you can run because I would hurt that guy."

So, who DOES Kamaru want to fight? Georges St-Pierre.

"If I had to choose, of course, I want Georges ... I want GSP."

Of course, St-Pierre officially retired from MMA earlier this year ... but there are rumblings he's down for one last scrap if the situation is right.

Kamaru says he has tremendous respect for GSP, but if the two get into the Octagon together, Usman tells us, "I think I'll finish him."

And, if you're still reveling in Usman's victory over Colby and wanna see Kamaru talk even more trash ... watch the clip below.

Kamaru says he's super proud for shutting Colby's mouth -- and get this, he explains why he still wants to take the UFC belt to the White House to meet Donald Trump.