1610s, "full of mystery, obscure, not revealed or explained," from Latin mysterium (see mystery (n.1)) + -ous. Related: Mysteriously; mysteriousness. Earlier in same sense was mysterial (early 15c.), from Late Latin mysterialis.
Mysterious is the most common word for that which is unknown and excites curiosity and perhaps awe; the word is sometimes used where mystic would be more precise. Mystic is especially used of that which has been designed to excite and baffle curiosity, involving meanings in signs, rites, etc., but not with sufficient plainness to be understood by any but the initiated. [Century Dictionary]
God moves in a mysterious way,
His wonders to perform;
He plants his footsteps in the sea,
And rides upon the storm
[Cowper, from the "Olney Hymns," 1779]
Related: Mysteriously; mysteriousness.