LMFAO -- Rick Ross Is a FAKE Thug!

LMFAO just made a really interesting legal argument ... slamming Rick Ross, and it could get them killed -- kidding ... maybe.

The group -- which Ross is suing for lifting his lyrics -- is taking a dangerous shot at the rapper, scoffing at the fact that he's a former corrections officer who built a fake, bad-ass persona strictly to gain cred with the rap community.

Here's the thing about Rick Ross. LM F'd with the wrong guy.

Ross sued LMFAO in December ... claiming the group stole his lyric, "Everyday I'm hustlin'" from his 2006 track "Hustlin'." Ross acknowledges they changed the words slightly to "Everyday I'm shufflin'" for their big hit, "Party Rock Anthem" -- but he says they couldn't mask the obvious thievery.

Now LMFAO just fired back in new legal dcocs, admitting they riffed off of Ross' lyric, but claiming it's fair game -- all part of the music biz.

We're waiting for Rick Ross' response ... well, his legal response.