Common abbreviation of リュックサック. Refers to a backpack or rucksack, typically worn on the back.
See also: リュックサック
A backpack is convenient for traveling.
He goes to school carrying a backpack on his back.
He started climbing with a big backpack on his back.
I put water and a lunch box in my backpack.
I took off my backpack on the train and put it on my lap.