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.