Origin: Latin suffix -ous
Mysterious has 3 different meanings across 1 category:
of an obscure nature
"the new insurance policy is written without cryptic or mysterious terms"
"a deep dark secret"
"the inscrutable workings of Providence"
"in its mysterious past it encompasses all the dim origins of life"
"rituals totally mystifying to visitors from other lands"
Of unknown origin.
"The origins of the ancient ruins remain mysterious to archaeologists today."
The word mysterious comes from the Old French term for "secret" or "hidden," which itself was borrowed directly from a Greek root meaning something reserved only for initiates in religious rituals. It entered English through Middle French and Latin before being adapted with an "-ous" suffix to describe things that are difficult to understand or explain.