1. difficult or impossible to understand, explain, or identify
2. having a quality that arouses curiosity or suspense because of an unknown or secretive nature

Examples

• The detective investigated the mysterious disappearance of the jewelry.
• She gave him a mysterious smile and walked away without saying anything.
• Strange, mysterious sounds came from the old house at night.
• The ancient ruins held many mysterious secrets.
• He was a mysterious figure who rarely spoke about his past.
• The mysterious illness baffled doctors for months.
• There was something mysterious about the way she avoided answering questions.