Rousey and Cyborg have been smack-talking each other for months ... with Rousey recently calling Cyborg a "fraud" ... and saying her failed drug tests in 2011 "nearly killed women's MMA."
Now, Cyborg is striking back ... telling TMZ, "Fraud? I’m not the one that hides behind a weight class that she knows I can’t make."
FYI -- Rousey has made it clear ... the only way she will fight the 145-pound Santos is if Cyborg drops down to the 135 lb weight class -- where Ronda currently reigns supreme.
Cyborg's response? "Meet me at 140 lbs" ... adding, "Then ask her if I’m a fraud after I knock her ass out!”
Cyborg -- who'll get in the ring for the Invicta Fighting Championships on April 5 -- says Rousey's ultimatum is the move of a coward:
"Obviously she knows that she can’t compete with me – take the time, maybe [Ronda] will get good enough and gain some confidence."
"Until then … I am the best and do not want to talk about hypotheticals. I fight in Invicta and I’m proud to be helping save women's MMA!"