大毘盧遮那成佛經疏 10.120

10.120

若不死者則亦不生。不生不死者是名甘露。世人以月能除毒熱煩惱同於甘露。故以為名。一切甘露之味無過淨月三昧也。

If there is no death, then there is also no birth. That which is unborn and undying is called amṛta. Worldly people consider the moon able to eliminate the afflictions of poisonous heat just like amṛta; hence it is named after it. Among the flavors of all amṛtas, none surpasses the pure moon samādhi.