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 LaDarius Marshall of the popular Netflix show, "Cheer" and captioning it," Derek F** Brunson.