A cryptic crossword.
"I spent hours trying to solve that particularly cryptic crossword clue about a silent movie star."
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"
having a secret or hidden meaning
"cabalistic symbols engraved in stone"
"cryptic writings"
"thoroughly sibylline in most of his pronouncements"
Having hidden meaning.
"The cryptic note left on my desk contained no name and hinted at a secret message I couldn't decipher."