marine (adj.)

mid-15c., "found in or pertaining to the sea," from Old French marin "of the sea, maritime," and directly from Latin marinus "of the sea," from mare "sea, the sea, seawater," from PIE root *mori- "body of water." The Old English word was sælic.

marine (n.)

late 14c., "seacoast;" see marine (adj.). Meaning "collective shipping of a country" is from 1660s. Meaning "soldier who serves on a ship" is from 1670s, a separate borrowing from French marine, from the French adjective. Phrase tell that to the marines (1805) originally was the first half of a retort expressing disbelief in some statement made or story told:

"Upon my soul, sir," answered the lieutenant, "when I thought she scorned my passion, I wept like a child."
"Belay there!" cried the captain; "you may tell that to the marines, but I'll be d----d if the sailors will believe it." ["John Moore," "The Post-Captain; or, the Wooden Walls Well Manned," 1805]

The book, a rollicking sea romance/adventure novel, was popular in its day and the remark is a recurring punch line in it (repeated at least four times). It was written by naval veteran John Davis (1774-1854) but published under the pseudonym "John Moore." Walsh records that, among sailors, marines are "a proverbially gullible lot, capable of swallowing any yarn, in size varying from a yawl-boat to a full-rigged frigate."

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Definitions of marine
marine (adj.)
of or relating to the sea;
marine explorations
marine (adj.)
relating to or involving ships or shipping or navigation or seamen;
marine insurance
Synonyms: nautical / maritime
marine (adj.)
of or relating to military personnel who serve both on land and at sea (specifically the U.S. Marine Corps);
marine barracks
marine (adj.)
relating to or characteristic of or occurring on or in the sea;
marine (adj.)
native to or inhabiting the sea;
marine plants and animals such as seaweed and whales
marine (n.)
a soldier who serves both on shipboard and on land;
Marine (n.)
a member of the United States Marine Corps;
Synonyms: devil dog / leatherneck / shipboard soldier