cock and bull story 
noun informal
An implausible story used as an explanation or excuse: nobody believes this cock and bull story about the sacking incident...
- It sounds like a cock and bull story, but he got into farming rare breeds through an interest in old furniture.
- Indeed, any of them who were informed would have been told the same cock and bull story that we were told at the briefing we had earlier today.
- I spoke with him yesterday and he was clearly upset by the cock and bull story somebody ran with on a local radio station.
Synonyms
fairy story/tale, made-up story, trumped-up story, fabrication, concoction, piece of fiction, fiction, falsification, falsity, lie, untruth, falsehood, fib, deception, barefaced lie;
(little) white lie, half-truth, exaggeration, prevarication, departure from the truth;
yarn, story, red herring, rumour, myth, flight of fancy, figment of the imagination;
pretence, pretext, sham, ruse, wile, trickery, stratagem
informal tall story, tall tale, whopper
British informal porky, pork pie, porky pie
humorous terminological inexactitude
vulgar slang bullshit
Australian/New Zealand vulgar slang bulldust