rucksack - Backpack issued to Grunts in Viet Nam. You carried ammo, water, three days food, poncho and liner, entrenching tool, machete, and squad stuff, like machine gun ammo, cans of M-16 ammo, flares, claymore mines, spare radio batteries, etc.  Most rucks weighed between 60 and 90 lbs.

The first rucksack I carried was in internal frame unit like the one shown below.  Good, but the rivet on the cross frame in the back could rub blisters.

The second rucksack was much better and had an external frame, allowing for much more flexibility in tying on extra squad stuff.

We usually put the sack up top on the frame, saving the lower part for poncho and squad stuff.