cock-and-bull story

An absurd or highly improbable tale passed off as being true.

noun

An untrue declaration: canard, falsehood, falsity, fib, fiction, inveracity, lie, misrepresentation, misstatement, prevarication, story, tale, untruth. Informal fish story, tall tale. Slang whopper. See true/false.

An unbelievable tale that is intended to deceive; a tall tale. For example, Jack told us some cock and bull story about getting lost. This expression may come from a folk tale involving these two animals, or from the name of an English inn where travelers told such tales. W.S. Gilbert used it in The Yeomen of the Guard (1888), where Jack Point and Wilfred the Jailer make up a story about the hero's fictitious death: "Tell a tale of cock and bull, Of convincing detail full." [c. 1600]

The noun has one meaning:

Meaning #1: an interesting but highly implausible story; often told as an excuse
  Synonyms: fairytale, fairy tale, fairy story, song and dance