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]

>-- [The Free Dictionary](http://idioms.thefreedictionary.com/fish+story)


(See also *bluster*, *blowhard*.)