|Noah Webster's Dictionary|
(a.) Of or pertaining to mystery; containing a mystery; difficult or impossible to understand; obscure not revealed or explained; enigmatical; incomprehensible.
Mysterious (2 Occurrences)
Ecclesiastes 7:24 Far off is true existence, and very deep; who may have knowledge of it? (See NAS)
Daniel 2:22 he reveals the deep and secret things; he knows what is in the darkness, and the light dwells with him. (See RSV)