magical circlet; tightening fillet
A fictional object from the Chinese classic Journey to the West (西遊記). It is a golden headband given by Guanyin to the monk Xuanzang to control the Monkey King Sun Wukong; when a special sutra is chanted, the band tightens and causes pain.
The 緊箍児 is a circlet placed on Sun Wukong's head that tightens when a sutra is chanted.