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 :-)
"If you
ain't a grunt you ain't SHIT"
"The grunts at Phase
Line Echo report multiple hostiles, scattered
small-arms fire and are currently engaging."
"The fuckin' grunts always come to our E-Club and start shit."