Why Would the Marines Want to Know?

Here's something you don't hear anymore:  "Tell it to the marines".  You may not have heard of it at all unless you're a big Lon Chaney fan and love his 1926 starring vehicle .... Tell It to the Marines.  The origin of this pithy phrase is found in the history of the British Royal Navy.  In the 1600's marines weren't sailors, they were landlubbers stationed on ships. What did they know?  Bupkiss.  So the actual sailors could tell the marines anything and the poor guys would believe it.  "Right, you shimmy on up the really tall pole there, that's where we keep the grog.  Keep going!  Almost there!"  The phrase came to be used as a rejoinder to anyone who recounted a big fat bag of lies.  Along the lines of, "No sir!".

Source:  Endangered phrases : intriguing idioms dangerously close to extinction / Steven D. Price.