/ mɪˈstɪəriəs; NAmE -ˈstɪr-/ adjective
1.
difficult to understand or explain; strange :
He died in mysterious circumstances.
A mysterious illness is affecting all the animals.
2.
( especially of people ) strange and interesting because you do not know much about them
SYN enigmatic :
A mysterious young woman is living next door.
3.
( of people ) not saying much about sth, especially when other people want to know more :
He was being very mysterious about where he was going.
► mys·teri·ous·ly adverb :
My watch had mysteriously disappeared.
Mysteriously, the streets were deserted.
She was silent, smiling mysteriously.
► mys·teri·ous·ness noun [ U ]
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WORD ORIGIN
late 16th cent.: from French mystérieux , from mystère mystery.