The Aussie rapper came out to address a report by blogger/YouTuber Tasha K ... who posted a new vlog this past week with the bold claim that Iggy has been sleeping with Tristan -- this amid other reports that Khloe Kardashian apparently wants him back (which she's denied).

Tasha flat out says ... "Does (Khloe) know that Tristan and Miss Iggy are f******?" She didn't elaborate on where she was getting her alleged intel -- but IA herself has shot it down.

She took to Twitter Sunday to deny the rumor ... writing, "Very weird fake news making the rounds about me dating a man I've literally never met a DAY in my life…. Like…. Y'all are really that bored?!?!"

Iggy went to add, "Literally have never even been in the same building as that man. I DO NOT KNOW THAT PERSON" -- this after a Twitter user pointed out it was Tristan whom she was referring to ... and followed up by saying Tasha seems to have made it all up.

The news does, in fact, seem to have come out of nowhere ... before Tasha came out with her so-called tea -- there wasn't even a whiff of this anywhere ... as far as we're concerned.

Yes ... Iggy has dated an NBA player in the past -- namely, Nick Young, who she was engaged to for a while back in the day before they split -- but there's no clear evidence, photos or otherwise, to indicate she or Tristan have ever met or mingled.