Webster's Revised Unabridged Dictionary · 1913

Mysterious

Mysterious , adjective

[French mystèrieux. See 1st Mystery.]

Of or pertaining to mystery; containing a mystery; difficult or impossible to understand; inexplicable; obscure; not revealed or explained; enigmatical; incomprehensible.
God at last To Satan, first in sin, his doom applied, Thought in mysterious terms. — Milton