"Moja siostra kupuje zielony plecak."

Translation: My sister is buying a green backpack.

[Jack.Elliot]

rucksack (ˈrʌkˌsæk)
n
a large bag, usually having two straps and a supporting frame, carried on the back and often used by climbers, campers, etc. US and Canadian name: backpack
[C19: from German, literally: back sack]

[Jellei]

Added to possible options.

[Jack.Elliot]

cheers

you are very helpful

thanks

[Jellei]

Happy to help :) Please don't edit your posts in the future, this "." creates a feeling of chaos, like what are we even talking about? You proposed something that made sense, we accepted it, let your proposition stay.

[Jack.Elliot]

cheers I will return that post

[JerryMcCarthy99]

Also "Knapsack" (with silent 'k'). Just sayin'...

[Jellei]

Added.