Woody Harrelson Says He Has 'Definite Confirmation' His Dad Hooked Up With Matthew McConaughey's Mom

Woody Harrelson says he's 100% sure his dad and Matthew McConaughey's mom did the deed ... claiming he's got a "definitive confirmation."

The two actors appeared on Travis and Jason Kelce's "New Heights" podcast to promote their new show "Brothers" -- which premiered on AppleTV+ Wednesday.

If you don't know ... the show is inspired by an oft-repeated story the two men tell -- which basically involves Matthew's mom, Kay, saying she "knew" (wink-wink) Woody's dad, convicted assassin Charles Harrelson, back in the day.

Woody and Matthew say she says "knew" with a whole lotta sexual innuendo and pause ... meaning the pair might actually be related, though Matthew has always avoided taking a genetic test.

Now, Woody says he's got "definitive confirmation" that the two were at least "bumping uglies" ... though he hasn't revealed what that might be to the Kelce bros.

Woody says the two are even similar in small ways ... with Woody saying even before the pair knew they could be related, he'd often see Matthew do something and feel he had a similar habit or tick.