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c.1600 (implied in hermetically), "completely sealed," also (1630s) "dealing with occult science or alchemy," from Latin hermeticus, from Greek Hermes, god of science and art, among other things, identified by Neoplatonists, mystics, and alchemists with the Egyptian god Thoth as Hermes Trismegistos "Thrice-Great Hermes," who supposedly invented the process of making a glass tube airtight (a process in alchemy) using a secret seal.
hermetic her·met·ic (hər-mět'ĭk) or her·met·i·cal (-ĭ-kəl)
Completely sealed, especially against the escape or entry of air.