mysterious
adj.adj. strange and difficult to understand or explain. You use this to describe something that has a secret or hidden quality.
adj. difficult or impossible to understand, explain, or identify. Often implies a sense of intrigue, secrecy, or a deliberate withholding of information.
A mysterious man left a letter at the door.
The detective spent years trying to solve the mysterious disappearance of the local baker.
The ancient ruins were covered in mysterious symbols that had baffled archaeologists and linguists for over a century.
From Middle English *misterius (implied in misteriusly), probably partly from Middle French mystérieux and partly from Latin mystērium + -ous. By surface analysis, mystery + -ous.
Typically used as an attributive adjective before a noun or as a predicative adjective after a linking verb.