Wikipedia claims UK military uses the term, I added that only from there. It's definitely common in US military lingo and the military-wannabes (see the many civilians or law enforcement who buy GoRuck rucksacks and attend Go Ruck events). I probably should've limited my comment to US-only, but went with what Wikipedia said and added it.
> not sure how it entered the US/UK military lingo
'Ruck' is definitely not UK military lingo! Talking about a 'rucksack' sounds extremely civilian to my British ears.
The UK military term is 'bergen' or 'daysack' or something smaller.