1
general::
Phrase(s): Tell it to the marines!
Inf. I do not believe you (maybe the marines will)! • Your excuse is preposterous. Tell it to the marines. • I don’t care how good you think your reason is. Tell it to the marines!
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms