Rick Ross to 50 Cent -- It's Not My Fault ... You're a Loser!

Rick Ross is getting personal in his legal battle with 50 Cent ... saying the only reason 50 sued him is to cover up the fact he's just not as famous as he used to be.

As TMZ first reported, 50 sued Ross to cover his own ass in a lawsuit he's facing over a leaked sex tape ... claiming it is Ross' fault the video was posted in the first place ... so Ross should pay, not him.

But Ross -- through his lawyer, Xavier Donaldson -- says 50's only gunning for him in "another attempt to draw attention to his descending entertainment career and one of several desperate, last minute attempts he's made to avoid and deflect legal and financial responsibility for his own actions."

Damn.