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]