G.I. grub

MRE

This military meal is ready to eat and often comes in a durable pouch.

"G.I. grub" refers to food provided to soldiers, with "G.I." being a common term for U.S. military personnel. "Grub" is slang for food, so the clue hints at military rations. The answer "MRE" stands for "Meal, Ready-to-Eat," the standard field ration used by the U.S. armed forces. The clue plays on the informal terms soldiers use for their food supplies.