UFC middleweight Paulo Costa crossed the line when calling out Derek Brunson for a fight ... referring to the fighter as a gay slur -- and now, he's getting bashed over it.
30-year-old Borrachinha has been going scorched-earth on social media recently ... pulling out all the stops in hopes of getting Brunson to get in the Octagon with him.
Costa -- who is 0-2 in his past two fights -- seemingly mocked Brunson for being a former cheerleader ... posting a pic of La'Darius Marshall of the popular Netflix show, "Cheer" and captioning it," Derek F** Brunson. Fight me bitch."
Costa's Twitter followers rightfully called him out for the slur ... saying, "You’re gonna end up getting canceled" ... and "@ufc seriously? This is ok from an employee of yours?"
There's more -- "Aside from being a bigoted and just low IQ middle school level insult, it's pretty rich coming from a guy who got styled on then literally humped by a dude who did Naruto hand signs."
The latter tweet is referencing Paulo's loss to Israel Adesanya ... who then humped his back after the 2nd-round TKO.