shaggy-dog story

noun

1. 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.

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]