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Scott Disick is not happy with reports that Kourney Kardashian is hooking up with Justin Bieber, but he also knows he's not in any kind of position to say anything.

"Scott feels she's doing this to get back at him and get revenge," a source tells Us Weekly. "He feels he has to take it and can't say anything after all he did to her, but he thinks it's dumb and immature."

That being said, he allegedly hasn't said a word to his former love about the romance.

Over the weekend, reports surfaced that the mother of three and The Biebs had been casually hooking up, news that seemed equally tantalizing and bizarre. Quite frankly, the news seemed hard to believe until pictures emerged of Kourtney leaving Justin's hotel at 4 am following an evening together on Dec. 11.

"Justin and Kourtney have been hooking up since late August, early September," the Us Weekly source said.

The news, it seems, got out after Justin "bragged" to his buddies that he's been hitting the sheets with the eldest Kardashian. Kourtney has denied that they've done anything outside of hanging out together in big groups.

Us Weekly says that's not true. "Justin started hanging out with Kourtney alone because she started going out more," the source said.

Over the summer, Kourtney and Scott split after a nine-year relationship. The split came after he was spotted getting a little too friendly with an ex-girlfriend in Monte Carlo. There had been other stories over the course of their relationship alleging that Scott wasn't faithful, although they were unconfirmed.

"He's had so many stories about him recently, he has no reason to be pissed," the magazine's source says. "He feels like he has to take the humiliation."

For now, though, he and the Kardashian family are still on good terms. In fact, he's sleeping on Kourtney's couch as he searches for a new place to live. The once permanent, but now temporary home is giving him a chance to spend more time with the children he and Kourtney share.

"He's still sober and he's great with the kids, and Kourtney is happy that he's so involved," a source says. "He's more involved now than he was before."