Say what you want about Rihanna's new "THUG LIFE" tat -- because she's already getting props from "Machete" star and tattoo connoisseur, Danny Trejo ... who calls RiRi the "most beautiful thug in the world."

Danny was in the LA tattoo parlor last night when Rihanna came in to get her art -- and tells us she "didn't stop saying thug life the whole time she was getting it done."

Danny -- who was getting his own impressive back tat of his kids touched up -- says he has no problem with Rihanna's Tupac tribute because she is "one of the downest female thugs in the world." A sentiment he must have been feeling when he and Ri took this pic.


Danny says he just kept thinking, "If I was 40 years younger she would be in a lot of trouble."

Rihanna must be a huge Tupac fan ... because she just tatted the words THUG LIFE -- on her fists!

RiRi hit up an LA tattoo parlor yesterday to get her hardcore ink started ... it doesn't look complete in the photo she tweeted out early this morning. It could be a temporary tat, but still ... badass.

Interestingly, the t-shirt she was wearing while she got inked up -- John Lennon's face. Totally thug.

Rihanna also ran into Danny Trejo at the tattoo parlor. Hope she's not inspired by his massive piece of chest art.