大毘盧遮那成佛經疏 13.72

13.72

此名無堪忍大護。由彼威光猛盛，如初生小兒不堪視烈日之光。此亦如是，一切不能堪忍而敢映奪之者，故名無能堪忍大護。以此護真言之行者也。

This is called the Great Protection That None Can Withstand. Because its awe-inspiring radiance is fierce and intense, just as a newborn infant cannot bear to look at the blazing light of the sun, this is also like that: none can withstand it or dare outshine it, so it is called the Great Protection That None Can Withstand. One protects the mantra practitioner with this.