The comedian dropped this would-be bombshell on his latest podcast episode, claiming he spoke with a Malibu grocery store employee recently, asking why they were closed one day in the week -- and he says the worker told the KarJenners were filming there.

Specifically, Tosh says this employee claimed they were filming the finale for the upcoming season of "The Kardashians" ... and during staged grocery store shopping being done by the fam, KJ apparently reveals she's expecting again -- which would make this her third kid.

The bigger headline here that Tosh is floating is that her boyfriend Timmy is the dad -- this while noting he doesn't know for sure if it's true or not ... saying this is just what he heard.

As it turns out, the story isn't true -- this according to production sources who tell us "The Kardashians" hasn't filmed at a grocery store once for the upcoming season. We're also told by sources with direct knowledge that the notion Kylie is pregnant via Timmy is false.

In fact, our sources tell us Kylie isn't expecting, period. We're told people familiar with the situation feel like Tosh put this out there for clout ... and that it was reckless for him to do so.

What's interesting is Kylie hasn't been doing anything to indicate she may be preggo -- in fact, a recent clip of her taking a sip of her new alcoholic seltzer drink, Sprinter suggests the opposite.

We also haven't seen Kylie with Timothée very much at all lately ... and she even refused to address their relationship in an interview she did a few weeks ago. Tim's been real busy lately -- promoting one movie, and starting to film the next ... the Bob Dylan biopic.

Obviously, if this would-be news was true -- it'd be huge. She already shares two children with Travis Scott ... and if there's another bun in the oven with Timmy ... well, that's mega. Unfortunately for Tosh and everyone else, though ... there's no there-there for this claim.