大毘盧遮那成佛經疏 10.46

10.46

此真言體也，無怖畏義也。訶是行，上一點是大空，下畫是定，所謂大空行三昧，即是大金剛三昧之異名。以能降伏，更無勝者故也。

This is the essence of the mantra, having the meaning of fearlessness. Ha is practice; the dot above is great emptiness; the stroke below is samādhi; the so-called great emptiness practice samādhi is an alternative name for the great vajra-samādhi. Because it can subdue and none can surpass it, it is so.