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Cardi B put her rear end into drive at a pop-up event in New York City ... throwing it back atop a black SUV while hundreds of fans swarmed around her.

The rapper held an event at a Washington Heights bodega on Saturday ... which brought out fans just hoping to catch a glimpse of the megastar.

They got more than a glimpse ... 'cause Cardi jumped on top of a car and danced around for the raucous crowd in a gray sweater and blue knee-high skirt -- and, she started dropping it low to applause from the audience.

Check out the vid ... Cardi's got her hands on her knees, and she's backing it up like only she knows how -- occasionally flattening her skirt so it doesn't raise up too high and reveal more than she wants to show. Gallery Launch Gallery Getty

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