Extract from OED Online

shaggy, a.

SECOND EDITION 1989

3. 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; also shaggy dog yarn, etc.

[1945 D. LOW in N.Y. Times Mag. 4 Feb. 40/1 The logical lunacy of 'Shaggy Dog'.] 1946 Coll. Shaggy Dog Stories facing p. 1 Stories of the Shaggy Dog variety are essentially tales to be told rather than read. 1947 Beat Apr. 6/3 Here's one of my favourite 'shaggy dog' stories. 1952 A. R. K. BARNARD in A. Redman Somewhat 'Shaggy' 4 The comparatively recent type of story

the 'Shaggy Dog' yarn. 1952 KOESTLER Arrow in Blue I. viii. 68 The people of Budapest have a peculiar shaggy-dog kind of humour. 1958 Listener 16 Oct. 623/1 It was a shaggy-dog story about a small-town worthy who shams madness to avoid paying bills. 1972 P. RUELL Red Christmas xi. 102 He seemed to be in the middle of an autobiographical shaggy-dog story.