He died in mysterious circumstances. Full of, characterized by, or involving mystery. Someone or something that is mysterious is strange and is not known about or understood.
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This is reminiscent of the extraordinary relationship which amputees develop with their artificial replacements, revealing a whole inner mysterious. The meaning of mysterious is of, relating to, or constituting mystery. See examples of mysterious used in a sentence.
Inscrutable applies to that which is.
That which is mysterious, by being unknown or puzzling, excites curiosity or awe: How to use mysterious in a sentence.