Kevin Hart Says His Nickname's 'The Rock,' Claims Dwayne Johnson Stole it

Kevin Hart says he's known around the gym as "The Rock" for his sculpted body ... and he's claiming Dwayne Johnson stole the famous handle and pawned it off as his own!!!

We got Kevin Monday in New York City and asked about getting in great shape for some recent movie roles, and that's when Kevin dropped a bombshell ... Dwayne's NOT the original 'Rock.'

Ya gotta see Kevin's reaction when our guy suggests the comedian needs a cool nickname like Dwayne's ... he's visually shocked and upset, imploring Dwayne he's got some 'splainin' to do!!!

We also wanted Kevin's take on Chris Rock crossing over to horror films and signing up to produce the next "Saw" chapter ... and gauged Kev's interest in getting involved with scary movies himself. Sounds like Kevin's got some demons to face before going down that road.