The Collaborative International Dictionary of English v.0.48:

leatherneck \leath"er*neck`\, n.
   1. (Zool.) The sordid friar bird of Australia
      (Tropidorhynchus sordidus).
      [1913 Webster]

   2. [from the leather collar that was once part of their
      uniform.] A member of the United States Marine Corps; a
      United States Marine. [slang]
      [PJC]

WordNet (r) 3.0 (2006):

leatherneck
    n 1: a member of the United States Marine Corps [syn: Marine,
         devil dog, leatherneck, shipboard soldier]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "leatherneck":
   Marine Corps, Naval Reservist, Royal Marine, Royal Marines, Seabee,
   bluejacket, boot, cadet, devil dogs, frogman, gob, horse marine,
   jollies, jolly, leathernecks, marine, marines, midshipman,
   midshipmite, naval cadet, navy man, sea soldiers, swabbie