‘Fish story’

A fish story is a tall tale. In other words, a great big lie.
Examples: Politicians are good at telling fish stories. *  “Do you seriously expect me to believe that fish story?”

Origin: Fishermen always have interesting stories to tell about the fish that they didn’t catch. In the stories that they tell, the fish that gets away is always very big. Hence the expression `fish story’.

Source: ‘Know Your English’ ( The Hindu) – June 11, 2007