1610s, "full of mystery," from Latin mysterium (see mystery (n.1)) + -ous. Related: Mysteriously; mysteriousness. Earlier in same sense was mysterial (early 15c.), from Late Latin mysterialis.
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]