Term of endearment applied by US Marines to their platoon-level medical personnel based on "Devildog". Unlike the other services the Marine Corps does not have a medical field, so we get ours from the Navy. A Hospital Corpsman is assigned to the platoon and he eats, sleeps, shits, fights and sometimes dies with "his" Marines.
A Corpsman is probably the single most highly respected member of the US Navy to an infantryman because he chooses to be there instead of some soft-ass REMF job. We treat them like a mascot and fuck with them constantly; within the platoon he's harassed about his branch of service, uniform, buddies, car, parents, girlfriend (or lack thereof), haircut (or lack thereof) and any manner of other things you know will rile him up.
Corpsmen tend to be highly protective of their Marines and there are many accounts of "Doc" coming to the aid of a wounded Marine under heavy enemy fire. John "Doc" Bradley is one example, winning the Navy Cross a short time after helping to raise the flag on Iwo Jima (he's one of the gentlemen immortalized on the monument).
The "Devildoc" is poked, prodded and ridiculed within the platoon (all in good fun, mind you), but let someone from outside the unit say one negative word about him and they can stand the fuck by. Any Marine worth his salt will buy a Devildoc a drink at the drop of a hat whether he knows him or not.
Also used when you're trying to buddy up to the Corpsman in order to get something from him.
Marine - "OORAH Devildoc!"
Corpsman - "Yeah, yeah... Whatever, Jarhead."
Marine - "Hey there, Devildoc. Wanna souvenir me one of those fuckin' cigarettes?"
Corpsman - "Golly gee, sure thing Marine! Take the pack!"
Corpsman *to himself* - "Broke-ass motherfucker..."