Secret

private undercover covered disguised mysterious dark obscure unknown covert classified furtive ambiguous recondite unenlightened camouflaged cryptic esoteric veiled arcane abstruse enigmatical unintelligible backdoor shrouded hush-hush underground unpublished unseen reticent close deep clouded retired mystic mystical secluded strange unfrequented cloak-and-dagger conspiratorial on the QT out-of-the-way under wraps

secretive covert stealthy sneaky sly furtive surreptitious underhanded discreet confidential classified restricted inside hidden undercover unacknowledged under the table hush-hush incognito disguised camouflaged dissembled dissimulated backdoor backstairs cryptic in ambush top secret under false pretense