Shaggy dog story
From bartleby.com
The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
shaggy-dog story
SYLLABICATION: shag·gy-dog story
PRONUNCIATION: shg-dôg, -dg
NOUN: Informal A long, drawn-out anecdote ending with an absurd or anticlimactic punch line.
OED: shaggy dog story, a lengthy tediously detailed story of an inconsequential series of events, more amusing to the teller than to his audience, or amusing only by its pointlessness...[first citation, Feb. 1945].