Khloe Kardashian and David Letterman had Kim's booty on the brain last night.

Khloe, Kim and Kourtney were all guests on Dave's show Tuesday night, where talk quickly turned to, what else, Kim's famous posterior.

While she was the one who likely supplied Letterman with her now famous butt X-ray, Kim looked peeved when Khloe started making fun of her for it -- saying "That's just fat. That's where all of her fat goes, just to her ass. That's it. It doesn't go anywhere else!"

Khloe wasn't done though, adding "it just sits and just goes right there. It's so frustrating. All she does it eat junk food and it goes to her booty."

To her credit, Kim says she works out a lot ... but finds the nuts on NY street corners irresistible.

Check out the pretty funny video above!

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