Something mysterious is strange and impossible to explain — for now. A mysterious light, a mysterious illness, mysterious circumstances.
It suggests a puzzle rather than a threat, and often a slight pleasure in not knowing. Of a person it means deliberately secretive: he was very mysterious about where he had been. Mysterious circumstances is a fixed phrase in news reports meaning the police have not said. Note mystery, the noun, which also names a genre of fiction; mystique, which is glamour built on not being known; and mysteriously, the adverb.
1adjectivemysteriousStrange and impossible to explain.
“A mysterious light appeared over the estuary for three nights.”
synonyms: puzzling, inexplicable, enigmatic
2adjectivedeliberatelyDeliberately secretive.
“He was mysterious about the whole trip and told nobody.”
Too few real examples of this word to estimate which sense is commonest.
mysterious is a quality of being unexplained.
Greek mysterion meant a secret rite, from mystes, one who has been initiated — and behind that a verb meaning to close the lips. The word's oldest layer is somebody keeping quiet, which is exactly what a mysterious person is doing.
Middle French
mystérieux
Middle English
*misterius
English, today
mysterious
From Middle English misterius (implied in misteriusly), probably partly from Middle French mystérieux and partly from mystērium + ous. by surface analysis, mystery + -ous.
Etymology from Wiktionary, CC BY-SA