大毘盧遮那成佛經疏 10.150

10.150

次守護者真言。此即是不可越，正為難持（謂力持義，觀瞻不可得也，亦有難降伏義）法佛奉教者常在內門右邊也，名為不可越使者，諸佛三昧耶威力不可過越也。金剛之戒奉以行之，一切不敢違越也。亦名無能見者，由此奉教者威猛熾盛如百千日，無敢視者，猶如初生小兒不堪仰視日輪，故以為名也。常在佛內門，有所教命如說而行，故名奉教者。

Next is the mantra of the guardian. This is Aparājita, precisely "difficult to withstand" (meaning held by power, impossible to gaze upon, and also meaning difficult to subjugate). He is the kāṅkara of the Dharma Buddha, constantly to the right of the inner gate, named the messenger Aparājita; the power of the samaya of all buddhas cannot be transgressed. Receiving and practicing the vajra precepts, none dare violate or transgress them. He is also called "Unviewable," because this kāṅkara is as fiercely blazing as hundreds of thousands of suns; none dare look at him, just as a newborn infant cannot bear to look up at the solar orb; hence he is named after this. Constantly at the Buddha's inner gate, whenever there are orders he acts as told; hence he is called kāṅkara.