tell it to the marines

(Go) tell it/that to the marines.

  (American)
something that you say in order to tell someone that you do not believe what they have just said
Usage notes: A marine is a soldier who works on a ship. Marines were thought to be less likely to believe things that people told them because they had travelled the world and knew a lot.
You were here all day? Sure, you were - tell it to the marines.
See also: marine, tell

tell it to the Marines

Go fool someone else because I won't believe that. For example, He's a millionaire? Tell it to the Marines! This term originated among British sailors, who regarded marines as naive and gullible. [c. 1800]
See also: Marine, tell

Tell it to the marines

A scornful response to an unbelievable story. Beginning in the 17th century, marines were land forces who were stationed on ships of the Royal Navy. As landlubbers, they were understandably naive if not ignorant about life aboard a vessel and on the waves. Sailors took advantage and concocted outlandish stories that the marines swallowed hook, line, and sinker. Accordingly, any outlandish story heard on land or sea and recognized as bilge was greeted with the full rejoinder, “You may tell that to the marines, but the sailors will not believe it,” subsequently shortened over generations to “Aw, tell it to the marines!”
See also: marine, tell