Arnold Schwarzenegger & Maria Shriver -- The Secret Butt Grab [VIDEO]

And now, definitive proof Arnold Schwarzenegger and Maria Shriver are definitely maybe BACK TOGETHER -- Arnold totally grabbed Maria's butt last weekend ... and TMZ has video.

As we reported, Arnold and Maria were attending their daughter Christina's graduation at Georgetown ... and while people were milling around everywhere, Arnold casually cupped Maria's backside. It's subtle, but it happened.

FYI, Arnold has been obsessed with Maria's caboose since he met her in the '70s -- in fact, Arnold recently told a story on "60 Minutes" about how he told Maria's mother her daughter had a "great ass."