A series of lies or exaggerations; a false or improbable explanation: His whole alibi is a fish story
[1819+; fr the tendency of an angler to exaggerate the size of the catch]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with 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