Kylie Jenner can pretend like she's too shy off her juicy booty -- but we all know the truth.

Wearing skin-tight grey pajama pants and matching top, the 18-year-old stood on a vibrating workout machine and mugged for the camera so that she could post yet another dim-witted Snapchat videos.

As her body and booty shook, the 18-year-old said, "I'm shaking." As soon as the person filming it tried to zoom on her butt, Kylie said "Stop, stop, stop it!"

Uhh Kylie -- what did you think would happen when you stood on a machine that makes your whole body jiggle? Your ass gyrates all over the place!

Sorry Ky -- you can't pretend to be a wallflower when you've posted as many pics of that big bottom of yours as you have!