大毘盧遮那成佛經疏 10.123

10.123

此釋為噉雲也。是黑暗，即諸眾生垢障。能噉諸障而得自在，故以為名也。以初迷字為體，即我義也，無我也。

This is explained as "cloud-devourer." It is darkness, which is the defilement-obstructions of sentient beings. Being able to devour all obstructions and attain mastery is why it is so named. It takes the initial syllable mi as its substance, which means "I"—that is, "non-self."