a funny story, traditionally about a talking dog, that, after an often long and involved narration of unimportant incidents, has an absurd or irrelevant punch line.
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British Dictionary definitions for shaggy dog story
informal a long rambling joke ending in a deliberate anticlimax, such as a pointless punch line
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Other Idioms and Phrases with shaggy-dog story
A long drawn-out anecdote with an absurd or anticlimactic ending. For example, At first he had us laughing wildly at his shaggy-dog stories, but after the third or fourth we found them tiresome. The term alludes to a well-known series of such stories, which involved a talking dog. [c. 1940]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.