(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!" Yeah, right, like you're related to that famous astronaut. Tell it to the marines.

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.

(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’.