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1. Of, relating to, or being a religious mystery: mysterious and infinite truths.
2. Arousing wonder or curiosity, especially by being difficult to explain or understand: the mysterious disappearance of the relevant files.

[French mystérieux, from mystère, secret, from Latin mystērium; see mystery1.]

mys·te′ri·ous·ly adv.
mys·te′ri·ous·ness n.
Synonyms: mysterious, esoteric, arcane, occult, cryptic, enigmatic
These adjectives mean beyond human power to explain or understand. Something mysterious arouses wonder and inquisitiveness: "The sea lies all about us.... In its mysterious past it encompasses all the dim origins of life" (Rachel Carson).
What is esoteric is mysterious because only a select group knows and understands it: a compilation of esoteric philosophical essays. Arcane applies to what is hidden from general knowledge: arcane economic theories. Occult suggests knowledge reputedly gained only by secret, magical, or supernatural means: an occult rite. Cryptic suggests a sometimes deliberately puzzling terseness: His roommate left cryptic messages alluding to his whereabouts. Something enigmatic is mysterious and puzzling: The biography struggles to make sense of the artist's enigmatic life.
References in classic literature ?
A certain mysteriousness hangs around the quality of speed as it was displayed by the old sailing-ships commanded by a competent seaman.
This sort of mysteriousness, which is always so becoming in a hero, threw a fresh grace in Catherine's imagination around his person and manners, and increased her anxiety to know more of him.
After that, appalling things happened, and the mysteriousness of the morning was explained to Mary.
On the following evening she went up as before, with the same mysteriousness and the same precaution.
Then she lowered her eyelids again, shutting all mysteriousness out of the situation except for the sobering memory of that glance, nightlike in the sunshine, expressively still in the brutal unrest of the street.
He was impressed and interested by the mysteriousness of the effect.
She was mysterious, with the mysteriousness of living beings.
The very mysteriousness of such a cure made her the more confident of its efficacy.
The mysteriousness of his quickened heart-beats startled him.
This mysteriousness is mirrored in the turns of the prose, as the reader is allowed to belong to an enigmatic world and observe it with awe.
Its length and complexity require an attentive reader; however, the story has an engaging mysteriousness, enhanced by beautifully described scenes in the snowy forests of the kingdom and in Paris in the 1930s and 1940s.
One of his intensely atmospheric tonal brush drawings made around 1831 in Shoreham, his 'valley of vision', the drawing explores the mysteriousness of the nocturnal landscape ($250,000-$350,000).