Funny how language evolves. When I did this stuff in the Army I don't recall the word 'ruck' being used; maybe it was and I've forgotten.
The German word for 'backpack' is 'rucksack', which is where I think this term comes from (and I was stationed in Germany in the Army, FWIW). The German word 'Ruck' by itself just means 'back', so if you were going backward, a German would say you were going rückwärts.
In the long distance backpacking community a 'ruck' is an event, a gathering. ALDHA-West is the national organization for long distance backpackers, and they organize 'rucks' on a regular basis:
https://www.aldhawest.org/rucks
I did 'weighted training' for long backpacking trips without having heard the term 'rucking' --- because I don't think it was in use that way at the time. I guess it makes sense.
The idea for my weighted training was along the line of Josephus' famous line about the Roman Army; if I recall correctly it was "
Their drills were bloodless battles, so that their battles were bloody drills". Train well enough with weight before doing a long backpacking trip and your body adjusts to the trail more easily (never 'easy' but easier).
What was common in training for a climbing or scrambling trip was to use water as weight: hike up (and up) to some high point with containers of water in your pack, then dump the water out at the top to limit knee stress on the way down.
What is now called 'rucking' is a fine idea in context, and particularly if not taken to the point of too much joint stress.