岱 is the classical literary name for
泰山 (Tài Shān, Mount Tai), the easternmost and most revered of China's Five Sacred Mountains (
五岳, wǔ yuè). Emperors performed the
封禅 (fēngshàn) imperial sacrifice here. Du Fu's
望岳 ('Gazing at Mount Tai') opens with
岱宗 — knowing the name unlocks the line.