Middle English: from mere² (in the obsolete sense 'sea') + maid.

• marine from Middle English:

  The root of marine is Latin mare ‘sea’, the source also of mariner (Middle English), maritime (mid 16th century), and mermaid (Middle English). Marinate (mid 17th century) and marinade (late 17th century) are closely related, having originally been used of pickles and coming from a word for ‘salt water, brine’. Marines were originally any men serving on board a ship, but later 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, used to express disbelief. It may have 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. 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.