玄妙 traces directly to 《
道德经》 (the Dao De Jing) — chapter 1 reads
玄之又玄,
众妙之门 ('mystery upon mystery, the gateway to all subtleties'). Daoist and Buddhist commentators later fused the two characters into
玄妙 to name the deepest layer of metaphysical truth. The flavor of 'unspeakable depth' is still attached to the word today.