The nickname “Jarheads” for U.S. Marines originated during World War II, referring to the appearance of a Marine’s head sticking out from the high collar of the dress blue uniform, resembling a Mason Jar. Initially an insult, Marines have since embraced the term, and it has also come to refer to the distinctive ‘high and tight’ haircut common among recruits.
  

Key Points: 

1. “Jarhead” is a term dating back to World War II, derived from the way a Marine’s head looked in the dress blue uniform’s high collar.
2. What began as a pejorative term was adopted by Marines and is now a term of endearment within the Corps.
3. The nickname has also become associated with the Marine Corps recruits’ ‘high and tight’ haircut style.
4. The term gained widespread recognition through Anthony Swofford’s memoir and the subsequent film adaptation, both titled “Jarhead.”
5. The Marine Corps community views the moniker “Jarhead” as a source of pride.

Source: https://www.militarytimes.com/off-duty/military-culture/2023/11/07/how-marines-got-the-nickname-jarheads/