The Collaborative International Dictionary of English v.0.48:

Hermetic \Her*met"ic\, Hermetical \Her*met"ic*al\, a. [F.
   herm['e]tique. See Note under Hermes, 1.]
   1. Of, pertaining to, or taught by, Hermes Trismegistus; as,
      hermetic philosophy. Hence: Alchemical; chemic. "Delusions
      of the hermetic art." --Burke.
      [1913 Webster]

            The alchemists, as the people were called who tried
            to make gold, considered themselves followers of
            Hermes, and often called themselves Hermetic
            philosophers.                         --A. B.
                                                  Buckley.
      [1913 Webster]

   2. Of or pertaining to the system which explains the causes
      of diseases and the operations of medicine on the
      principles of the hermetic philosophy, and which made much
      use, as a remedy, of an alkali and an acid; as, hermetic
      medicine.
      [1913 Webster]

   3. Made perfectly close or air-tight by fusion, so that no
      gas or spirit can enter or escape; as, an hermetic seal.
      See Note under Hermetically.
      [1913 Webster]

   Hermetic art, alchemy.

   Hermetic books.
      (a) Books of the Egyptians, which treat of astrology.
      (b) Books which treat of universal principles, of the
          nature and orders of celestial beings, of medicine,
          and other topics.
          [1913 Webster]

WordNet (r) 3.0 (2006):

hermetic
    adj 1: completely sealed; completely airtight

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "hermetic":
   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