tell that to the marines

(go) tell it/that to the marines

A scornful or incredulous response to a story or statement that one does not believe or finds ridiculous. A: "You know, my dad used to play basketball with the president when they were both kids." B: "Ah, go tell it to the marines! Why do you tell such fibs?" A: "I bet you I could eat 20 hot dogs in less than half an hour!" B: "Tell that to the marines, pal!"
See also: marine, tell, that
Farlex Dictionary of Idioms.

tell that to the marines (or the horse marines)

a scornful expression of incredulity.
This saying may have originated in a remark made by Charles II , recommending that unlikely tales should be referred to sailors who, from their knowledge of distant places, might be the people best qualified to judge their truthfulness. Horse marines, dating from the early 19th century, were an imaginary cavalry corps, soldiers mounted on horseback on board ship being a humorous image of ineptitude or of people out of their natural element. In 1823 Byron noted that That will do for the marines, but the sailors won't believe it was an ‘old saying’, and the following year Walter Scott used Tell that to the marines—the sailors won't believe it! in his novel Redgauntlet.
1998 Times Truth is the issue, say the apologists, not the grope. You can tell that to the marines. The issue is the grope.
See also: marine, tell, that
Farlex Partner Idioms Dictionary

(go) tell it/that to the maˈrines

(saying, informal) used to say that you do not believe what somebody is saying, promising, etc: ‘I’ll never smoke again!’ ‘Yeah? Go tell that to the marines.’This comes from the saying ‘that will do for the marines but the sailors won’t believe it’.
See also: marine, tell, that
Farlex Partner Idioms Dictionary
See also:
  • (go) tell it/that to the marines
  • marine
  • tell it to the Marines
  • Tell it to the marines!
  • tell it/that to the marines
  • (Go) tell it/that to Sweeney!
  • cockamamie
  • Don’t make me laugh!
  • don't make me laugh
  • Don't make me laugh!
References in periodicals archive
"They can tell that to the marines but (I) still believe that it contained money because of their suspicious actions," said Yap, who claimed that the three were whispering to each other and were looking around, as if they were doing something dishonest that should be concealed.
Brillantes clears officials of misconduct charges
And if they suggest that this is not an attack on the Prime Minister, then you can tell that to the marines.
Time for Brown to rip into these Labour toy town nonentities; AGENDA
We need to find better ways of using the existing road network." Well, I'm sorry Mr Jamieson, tell that to the Marines. The transport policy of this and previous governments has been to have no meaningful policy.
On the road to nowhere
Tell that to the Marines, or if you don't like the Brits, try the UFF/UDA/UDP, or the UVF/PUP or even Sinn Fein/IRA.
John Hunter's column: Omagh bomb blame game is a Real sickener
Leyendecker (the lone, indomitable hero), Howard Chandler Christy (women as "other," or the opposite of the ideal Marine), and James Montgomery Flagg, who appeared on the steps of the New York Public Library in July of 1918 to redraw his pugnacious "Tell That to the Marines!" before a cheering crowd, in an audacious act of wartime street theater.
American Samurai: Myth, Imagination, and the Conduct of Battle in the First Marine Division, 1941-1951
It was so girlish it was embarrassing." Tell that to the marines.
HURL OF WESSEX