Welp, here's a scary threat from Kamaru Usman to Jake Paul -- cut the crap before you get hurt, for real.

Yeah.

Okay, here's the quick backstory ... Jake Paul has been looking for ALL THE SMOKE from UFC fighters after ferociously knocking out Ben Askren.

He's called out Conor McGregor, beefed with Tyron Woodley and went face-to-face with Daniel Cormier at UFC 260.

Kamaru -- the reigning UFC welterweight champ -- told TMZ Sports back in April he was getting sick of Paul's trash talk and said he would love the opportunity to "beat the s**t" out of him.

Paul responded to Usman, "Challenge accepted" -- adding, "If your boss Dana gives you permission to box me and make more money than you ever have, let me know and we can lace them up. I promise it will be your biggest pay day."

The two continued to trade shots over social media -- with Jake accusing Usman of "backing out."

Which led to Usman responding yet again on Wednesday morning ...

"I understand you think this is a game which is probably why you don’t realize that this is how people Truly get hurt," Usman said.

"I don’t play fighting and I can change your life in the worst way."

He's not kidding ... Usman is coming off one of the most violent KO's of the year when he flattened Jorge Masvidal with a killer right hand at UFC 261.

Usman's manager Ali AbdelAziz certainly wants a fight to happen -- telling us he thinks it would be one of the most successful PPV events in combat sports history.