dirty grundler
A Dirty Grundler is a certain kind of storyteller, known for telling tall tales and spreading false facts with a deadpan, straight-faced delivery, indistinguishable from if they had been completely serious and in earnest. A Dirty Grundler isn't a liar, per se - they aren't attempting to deceive anybody, just entertain.
A: Ray told me this is the longest saltwater river in North America!
B: Dude, Ray's a Dirty Grundler, there are no saltwater rivers in North America.
B: Dude, Ray's a Dirty Grundler, there are no saltwater rivers in North America.