Tyrann Mathieu says he did not tell Titans fans to "sit on a d***" after the Chiefs' loss Sunday ... calling the reports a straight-up "lie."
The Honey Badger allegedly unleashed the crass insult following K.C.'s 27-3 defeat at Nissan Stadium in Nashville ... with a local reporter tweeting he heard it loud and clear on the sidelines.
"I and another photographer heard him say those words as he turned around to some fans," K.C. sports anchor Harold R. Kuntz said.
Mathieu, though, was quick to deny the allegation ... writing on his own social media page (and then later deleting), "This is a lie!"
"I can’t believe this dude lied on me," Mathieu added in another tweet.
I can’t believe this dude lied on me.
— Tyrann Mathieu (@Mathieu_Era) October 24, 2021
Kuntz said he talked with Chiefs staffers over it all ... and while he regretted tweeting about it and apologized to Mathieu over it -- he stood by his claims.
"I am standing by what I heard, but I should not have tweeted it, in hindsight," Kuntz said. "It was a frustrating game but should not have shared that info on social. For this, I apologize."
Mathieu recorded 6 tackles in the game ... but it wasn't close to enough to help K.C. lock down a win -- and the Chiefs are now just 3-4 in the season despite heading into the year with Super Bowl hopes.