Synonyms and related words :
abstruse, airtight, arcane, ballproof, bombproof, bulletproof, burglarproof, cabalistic, censored, classified, cloistered, close, closed, compact, concealed, corrosionproof, cryptic, dampproof, dark, deep, dustproof, dusttight, enigmatic, esoteric, fast, fire-resisting, fireproof, firm, flameproof, foolproof, gasproof, gastight, heavy, hermetical, hermetically sealed, hidden, holeproof, hush-hush, impervious, impervious to, latent, leakproof, lightproof, lighttight, mysterious, noiseproof, occult, oilproof, oiltight, profound, proof, proof against, punctureproof, rainproof, raintight, recluse, resistant, restricted, rustproof, sealed, secluse, seclusive, secret, sequestered, shatterproof, shellproof, shut fast, smokeproof, smoketight, smothered, snug, soundproof, staunch, stifled, stormproof, stormtight, suppressed, tight, top secret, ulterior, unbreatheable, under security, under wraps, undisclosable, undisclosed, undivulgable, undivulged, unrevealable, unrevealed, unspoken, untellable, untold, unutterable, unuttered, unwhisperable, water-repellant, waterproof, watertight, weatherproof, windproof, windtight