An
improbable or
unbelievable story, often obviously so.
The term is said to be derived from the names of two coaching inns in the town of Stony Stratford in Buckinghamshire, England: The Cock and The Bull. Back in the day the two inns were staging posts on the London-Birmingham route. London coaches changed horses at The Cock, Birmingham coaches at The Bull. Passengers frequently got off and exchanged news and anecdotes, which were known to become somewhat exaggerated or dubious. The two inns are still there to this day.