The term "grunt" is used in the military as a general term for someone who's MOS (Military Occupational Specialty) is "Infantry". In the
Marine Corps all MOS' preceeded by the number "03" are Infantry. About as "grunt" as you can get in the Corps is "0311 - Basic Rifleman".
The opposite of a "grunt" is a "pougue", which is a derogatory
reference to
pretty much anyone who isn't a grunt, but normally reserved for Marines who work in an office or some other rear-echelon job as part of their regular duties ("In the rear with the gear"). Call a pougue a "grunt" and they love it, but call a grunt a "pougue" and see what happens :-)