Mysterious is a word used to describe something that is strange and unexplainable. However, if you want to find a more nuanced way to describe something as mysterious, there are many synonyms you can use. For instance, you might say that something is enigmatic, inscrutable, or arcane. Other possible synonyms include cryptic, impenetrable, or inexplicable. You could also describe something as mystical, eerie, or uncanny. In general, synonyms for the word "mysterious" tend to convey a sense of intrigue or wonder, suggesting that the object or concept in question is difficult to understand or explain but also fascinating and captivating in its ambiguity.