According to Heritage Press International: 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.