unknowable; mysterious
Describes something that cannot be known or understood, often in philosophical or abstract contexts. Can also be used more loosely for something deeply mysterious.
Many aspects of the origin of the universe are unknowable.
His true intentions remained mysterious.
神秘的 means 'mysterious' with a nuance of awe or spiritual wonder, whereas 不可知 is more about the limits of knowledge.
Compound of 不 (not) + 可 (possible) + 知 (know), literally 'not possible to know'. The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese pattern for forming abstract concepts.