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A student says she was humiliated when Joe Biden called her a "lying, dog-faced pony soldier" - an insult that Biden later claimed was a just a joke.

The incident happened during his campaign event in Hampton, New Hampshire on Sunday.

21-year-old Mercer University student Madison Moore asked Biden about his poor performance in last week's Iowa caucus.

Biden, 77, came in third in the voting behind Pete Buttigieg (1) and Bernie Sanders (2).

"How do you explain the performance in Iowa and why should the voters believe that you can win the national election?" Moore asked Biden.

"It's a good question," said Biden. "Number one: Iowa's a Democratic caucus," he said before asking the student, "Have you ever been to a caucus?"

When she nodded yes, Biden said, "No, you haven't. You're a lying dog-faced pony soldier."

Biden's insult was met with nervous laughter from the crowd.

Although Moore is seen laughing in the video footage, she later said she felt humiliated.

"It was kind of humiliating to be called a liar on national TV by the former vice president," the economics student said.

Biden's camp defended their candidate by claiming the line was from a John Wayne film in which an Indian chief turned to Wayne and called him a "lying, dog-faced pony soldier."

But movie historians say no such line was ever uttered in any John Wayne movie.

They think Biden - who likely suffers from dementia - combined several lines from different movies, including 1949's She Wore a Yellow Ribbon and the film Pony Soldier, starring Tyrone Power.

This is just the latest in a long series of goofs and missteps by the Democratic frontrunner.

Biden has been criticized in the past for insulting voters who ask him confrontational questions.

At a campaign event in Iowa in December, Biden called a man a "damn liar" after he brought up his son Hunter's role in the Ukraine scandal.

Iowa was the first state to hold a Democratic caucus.