玄之又玄 Origin — Chinese Character Wiki

xuánzhīyòuxuán

extremely mysterious

玄 xuán

profound

deep and mysterious; profound; abstruse
之 zhī particle

[intensifier]

Used between a subject and a following adjective to emphasize the degree or extent of a quality.
又 yòu adverbconjunction

and

Moreover; additionally; indicates a further step in meaning or an addition to a range
玄 xuán

profound

deep and mysterious; profound; abstruse