yay finally found it
https://www.phrases.org.uk/meanings/tell-it-to-the-marines.htmlA scornful response to a tall and unbelieved story.
[The commanding officer] if a soldier complained to him of hardships which he could not comprehend, would be very likely to recommend him to "tell it to the marines"!
The longer version of the phrase is found earlier, in John Davis's The Post-Captain, or, The wooden walls well manned comprehending a view of naval society and manners, 1804:
"He may tell that to the marines, but the sailors will not believe him."
I hadn't heard it before, but will use it whenever possible.
Many thanks for your interest.