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"mysterious" is also used as a crossword clue. If that's what you saw, the answer is often enigmatic (9 letters).
Definitions
Difficult to understand, explain, or identify.
Word origin
From Middle English *misterius (implied in misteriusly), probably partly from Middle French mystérieux and partly from Latin mystērium + -ous. By surface analysis, mystery + -ous.
Used in a sentence
“He received a mysterious phone call this morning.”
“Our boss is a mysterious man who hardly ever meets with us.”
“God moves in a myſterious way, / His wonders to perform; / He plants his footſteps in the ſea, / And rides upon the ſtorm.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Hard to identify”10 letters
02“Hard to explain”10 letters
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