Kourtney Kardashian -- Sun's Out Buns Out (PHOTO)

Kourntey Kardashian Sun's Out, Buns Out

Kourtney 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.