Jar is a cylindrical vessel with no handle and Jarhead is someone whose head is as empty as a jar, presumably. US Marines are called jarhead and as said in the movie, it's because they are dumb people who don't think and have to just follow the orders. I went to the movie last night with P. M., the genius (!) and that was not what I expected. The movie was a combination of Full Metal Jacket, Platoon, Saving Private Ryan and may be a few more war movies. Doesn't present anything new but a few desert scenes and strong coarse language. The film is not considered a comedy but you can laugh time to time your head off. That's the only good thing about the movie as you don't find it in the previous said ones.
(Photo: Jamie Foxx is seen here through a gas mask while directing the soldiers in a anti-chemical drill)