Wiktionary
shaggy-dog story
n. A long, rambling, pointless story or joke, especially one with an absurd punchline
Usage examples of "shaggy-dog story".
It was Tall Tales Night at Callahan's, the night on which the teller of the most outrageous shaggy-dog story gets his night's tab refunded.
Amused by the thought that the entire Talmud and the Old Testament might be nothing more than a cosmic shaggy-dog story.
It was, as far as they were concerned, a big shaggy-dog story with a duff punchline.