Military slang referring to infantry or Special Forces personnel, or paramilitary components of the US Intelligence Community.
The term stems from the common belief among the rear echelon and support personnel that Infantry soldiers are of generally lesser intelligence than support personnel.
The Liberty chow hall was full of knuckle draggers today, they smelled like B.O. and gun powder.
I had to catch a ride to the Green Zone with a platoon of knuckle draggers, and they made fun of my M16 A2 the entire time.