5:14 PM PT -- Tekashi is firing back at Fat Joe, telling TMZ ... "Fat Joe is jealous of 69. 69 has done more in his young career than Fat Joe has ever done in his long career."
6ix9ine adds ... "Fat Joe said I was miserable and want to die? Fat Joe is miserable and wants to die because Fat Joe doesn't have half of what 69 has in money and cars."
Tekashi concludes ... "Fat Joe said I'm not a gangster, it was all for promotion? That's fake news. What's his proof that I ever said that? There is no proof. He's lying."
Fat Joe can't hold his tongue when the topic of Tekashi 6ix9ine and snitching comes up -- and now he's tearing the rainbow-haired rapper a new soul.
Joey Crack was the latest guest of Math Hoffa's barbershop-style podcast "Expert Opinion," and he had some choice words ... describing Tekashi as "a pussy, a sucka, a bitch!"
If you recall, 6ix9ine infamously ratted on his Treyway Bloods Gang and justified the deed by saying they turned on him, but Joe couldn't give, pardon the phrase, a rat's ass!
As he started going in on Tekashi, Joe pointed out he was sober and not drunkenly spewing opinions. They both hail from NYC -- completely different eras -- and Joe hints at feeling betrayed when he saw firsthand Tekashi admitting he's just trolling and pretending to be a gangsta.
For his part, Tekashi recently claimed on "TMZ Live" he doesn't start all the beefs he has with various rappers, he just fillets them when they come his way.
He can add one more to that list, and while Fat Joe says he doesn't wish any harm on 6ix9ine ... he also believes the guy's got a death wish.