Learner's definition of MYSTERIOUS
[more mysterious; most mysterious]
: strange, unknown, or difficult to understand
the mysterious ways of nature
We heard a mysterious noise outside our tent.
He died under mysterious circumstances.
A mysterious illness has been spreading through the city.
There's something mysterious about that old woman.
Her behavior was very mysterious.
◊ Mysterious people are often interesting because many things are not known about them.
: to talk or behave in a way that makes other people feel that you must have a secret