The adjective cryptic means something hard to understand and mysterious in some way.

Synonyms are ambiguous, strange, secret, or vague.

The word origins from Late Latin (1630s) crypticus meaning “hidden, occult, mystical”. It also comes from Greek kryptikos meaning “fit for concealing”, from kryptos meaning “hidden”. The meaning “mysterious, enigmatic” is attested by 1920.

The response of the student was cryptic that made her hesitate.

Sam’s cryptic warning makes sense.

The short letter was too cryptic.