8/22 -- We've learned the drinking driver is Michael Pettersen, a prosecutor with the L.A. County D.A.'s Office who's been on a leave of absence since January 1, 2017.

It looks like a joke -- but this was very real.

Legendary radio DJ Big Boy was rear-ended by a VERY drunk driver in Calabasas ... and when he confronted the man, the guy pulled out a VODKA BOTTLE and started chugging while still behind the wheel.

It all went down Tuesday at 2 PM -- Big Boy was driving his Nissan Armada when he was smashed from behind.

The DJ appeared to be okay -- but couldn't believe his eyes when he saw the other guy pull out the bottle.

"He just crashed into me and he's in the car drinking," Big Boy said ... "He is still in the car, still drinking!"

A bystander told the driver to put the booze down -- but he refused.

Cops ultimately got to the scene and pulled the man out of his BMW -- his pants fell down ... and then he did, too.

We're told the man was transported to a local hospital where he was being checked for injuries and most likely undergoing blood alcohol tests (though you can guess what they're gonna show).

The area where the collision occurred is right near The Oaks -- one of the most prestigious, elite communities in L.A. where stars like Khloe and Kourtney Kardashian own homes.