fish story

An implausible, boastful story.

[From the fact that fishermen traditionally exaggerate the size of their catch.]

noun

An untrue declaration: canard, cock-and-bull story, falsehood, falsity, fib, fiction, inveracity, lie², misrepresentation, misstatement, prevarication, story, tale, untruth. Informal tall tale. Slang whopper. See true/false.

An improbable, boastful tale, as in He came up with some fish story about his winnings at the track. This expression alludes to the tendency of fishermen to exaggerate the size of their catch. [Early 1800s]