RUDE GORILLA GIVES GIRLS THE BIRD!

Sisters interrupt his celery snack

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BADASS Bob the gorilla has turned into an X-rated attraction at a Colorado zoo, where he's flipping off shocked kids!

The raunchy western lowland ape was caught giving the naughty middle-finger sign to stunned sisters Arabella, 11, and Arianna, 9, on camera by their mom, Amy, at Colorado Springs' Cheyenne Mountain Zoo.

Later, the ornery encounter was posted on YouTube, where it became a hilarious sensation.