box for carrying books on one's back
Historical term for a box-like container strapped to the back, used by traveling scholars or monks to transport books and documents.
Traveling monks of old carried sutras in a box on their backs.
pilgrim's backpack box
Specifically a wooden box used by pilgrims to carry belongings. The reading おい is more common for this sense; きゅう is a rare alternative reading.
See also: 笈 (おい)
The mountain ascetic carried a backpack box.