Maybe it was just a joke as both shoot their new movie, 'Dirty Grandpa'.
Robert De Niro played a rather bizarre game with young hunk Zac Efron which has left many fans confused.
The veteran hollywood superstar was caught 'jokingly' pushing his thump into Zac's butt while on set of their latest film, 'Dirty Grandpa'.
In the photo, Zac has absolutely no idea De Niro was up to that as he is seen standing and looking in a different direction.
Meanwhile De Niro is seen talking on his phone while he played with the younger actor.
Both stasr are currently on set of their new movie, which has seen the former Disney actor do really 'crazy' things including wearing a crop top and a nude underwear in public.
Wow! We think we now know who the 'dirty grandpa' is.