A jarhead is a US Marine: a member of the United States Marine Corps, the force trained to fight from ships onto land. Informal, American, military slang. It is not a Navy sailor, and it is not just any soldier. Dictionaries sometimes flatten Marine to “a soldier who serves on a ship”; though actually Marines are their own branch, and sailors in the Navy would not thank you for mixing them up. The name comes from the regulation haircut, the high and tight: sides cut very short, the top looking a bit like a jar sitting on the head. Among Marines it can be an in-group nickname, even said with a grin. From the outside it can sound rough or mocking, so people who are not in that world usually say Marine. You will hear it in films, in barracks talk, and in jokes about the haircut. It is not a formal rank and you would not put it on a form.

美國海軍陸戰隊員（俚稱）