Tell a fish story. That a boastful fib (lie), or at the very least a great exaggeration.
fish story and fish tale
Fig. a great big lie. (Like a fisherman who exaggerates the size of the fish that got away.)
That's just a fish story.
Don't try to fool me. He's a master at the fish tale. Maybe he should be a politician.
fish story
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]
(See also bluster, blowhard.)
