marine [ME] The root of marine is Latin mare ‘sea’, the source also of mariner [ME], maritime [M16th], and mermaid [ME]. Marinate [M17th] and marinade [L17th] are closely related, having originally been used of pickles and coming via French, Spanish, and Italian from aqua marina ‘salt water, brine’. Marines [L16th] were originally any men serving on board a ship, but later, from the late 17th century, the meaning was restricted to troops who were trained to serve on land or sea, now particularly the Royal Marines or, in the USA, the Marine Corps. These facts shed little light on the likely source of the expression tell that to the marines [L19th], used to express disbelief. There is a story that this begun with a remark made by King Charles II (1630–85). He advised that implausible tales should be checked out with sailors, who, being familiar with distant lands, might be the people best qualified to judge whether they were true or not. However, this was a 19th-century hoax. Another idea picks up a clue left in the longer version tell that to the horse marines. The horse marines were an imaginary troop of cavalry soldiers serving on board a ship, used as an image of total ineptitude or of people completely out of their natural element. The idea is that such people are so clueless that they will believe anything they are told. Byron wrote in 1823, “‘That will do for the marines, but the sailors won’t believe it’, is an old saying”, which may be closer to explaining it.