Joe Biden Calls Voter 'Lying Dog-Faced Pony Soldier,' Credits John Wayne

Joe Biden called a young female voter a "lying, dog-faced pony soldier" -- a line he's used before and has credited to a John Wayne movie ... which may or may not be accurate.

Biden was campaigning in New Hampshire Sunday, and while at a town hall event where he fielded questions from the crowd, one gal got on the mic and asked something pretty profound ... with his poor performance in Iowa, why should Joe be considered a top dog.

It was a great question from a prospective Biden voter, and right away ... Joe jokingly insulted her by suggesting she was lying. Check out the video, Biden asks the girl if she's ever caucused before. She says yes, but he calls her a liar with the alleged John Wayne line.

We should note ... it doesn't seem Biden was being serious. He said it jokingly and it came off as good-natured. BUT, folks were quick to point out ... it sounded somewhat inappropriate.

What's even crazier is that Joe has used this line before, and explained it in an interview as a line ol' JW used in a movie he saw once. Problem is ... nobody can pinpoint that flick.

It might be because the line Joe thinks was in the unnamed Wayne movie is probably actually from a 1952 Tyrone Power film called "Pony Soldier," which uses that phrase throughout. The whole movie's on YouTube, but we couldn't find this exact pony remark, if you will.