What does "cock and bull story" mean?

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]