a tall tale
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A greatly exaggerated or particularly imaginative story or account of something, such that it is difficult to believe. My uncle has always been fond of telling tall tales about his time overseas. I used to believe him when I was a kid, but now I've learned to just laugh along.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a tall taleBRITISH, AMERICAN or
a tall storyBRITISH
COMMON A tall tale or a tall story is a very exciting or extreme event that someone tells you about but that you think is probably not true. He was very convincing, and some of his college chums believed his tall tales. `She told my guests a tall story about our relationship,' he said. `She claimed we'd been in love three years.' Note: `Tall' used to be used to describe language that was considered extremely formal or exaggerated.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012