Refers to a cord or string used to close the opening of a bag, pouch, or similar container by pulling it tight.
巾着の口紐をぎゅっと引っ張って閉じた。
I pulled the drawstring of the pouch tight and closed it.
Compound of 口 (kuchi, 'mouth, opening') and 紐 (himo, 'string, cord').