Kourntey Kardashian Suns Out, Buns Out

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0320-kourtney-kardashian-butt-bikini-pretty-FACEBOOK-01Kourtney Kardashian was really feelin' it Sunday ... in a super skimpy bikini.

Interesting ... Kourtney posted the pic on social media a day after Justin Bieber posted a throwback of him with ex-GF Selena Gomez.

So at the risk of overthinking it, she's telling Biebs ... butt out.