What's in a name?

from Warrior Culture of the U.S. Marines

Devil Dogs
The German Army coined this term of respect for U.S. Marines during World War I. In the summer of 1918 the German Army was driving toward Paris. The French Army was in full retreat. In a desperate effort to save Paris, the newly arrived U.S. Marines were thrown into the breach. In June 1918, in bitter fighting lasting for weeks, Marines repeatedly repulsed the Germans in Belleau Wood. The German drive toward Paris sputtered, fizzled, and died. Then the Marines attacked and swept the Germans back out of Belleau Wood. Paris had been saved. The tide of war had turned. Five months later Germany would be forced to accept an armistice. The battle tenacity and fury of the U.S. Marines had stunned the Germans. In their official reports they called the Marines "teufel hunden," meaning Devil Dogs, the ferocious mountain dogs of Bavarian folklore.

Gyrene
Around 1900, members of the U.S. Navy began using Gyrene as a jocular derogatory reference to U.S. Marines. Instead of being insulted, the Marines loved it. The term became common by World War I and has been extensively used since that time.

Jarhead
For roughly 50 years, sailors had little luck in their effort to insult Marines by calling them Gyrenes. So, during World War II sailors began referring to Marines as Jarheads. Presumably the high collar on the Marine Dress Blues uniform made a Marine's head look like it was sticking out of the top of a Mason jar. Marines were not insulted. Instead, they embraced the new moniker as a term of utmost respect.   ...   Alternate explanation: the term originates from the "high and tight" haircut that many Marines have, which makes their head look like a jar.

Leatherneck
The nickname Leatherneck has become a universal moniker for a U.S. Marine. The term originated from the wide and stiff leather neck-piece that was part of the Marine Corps uniform from 1798 until 1872. This leather collar, called The Stock, was roughly four inches high and had two purposes. In combat, it protected the neck and jugular vein from cutlasses slashes. On parade, it kept a Marine's head erect. The term is so widespread that it has become the name of the Marine Corps Association monthly magazine, LEATHERNECK.