her·met·ic / hərˈmetik/ • adj. 1. (of a seal or closure) complete and airtight: a hermetic seal that ensures perfect waterproofing. ∎  insulated or protected from outside influences: a hermetic society.2. (also Her·met·ic) of or relating to an ancient occult tradition encompassing alchemy, astrology, and theosophy. ∎  esoteric; cryptic: obscure and hermetic poems.DERIVATIVES: her·met·i·cal·ly / hərˈmetiklē; -ik(ə)lē/ adv.her·met·i·cism / hərˈmetiˌsizəm/ n.

hermetic pert. to (the supposed writings of) Hermes Trismegistus; (hence) pert. to occult science, esp. alchemy; h. seal airtight closure (as used by alchemists). XVIII. — modL. hermēticus, f. Hermēs Trismegistus (Gr. Hermés trīs mégistos) ‘thrice-greatest Hermes’, name given by Neoplatonists, mystics, and alchemists to the Egyptian god Thoth, who was identified with the Grecian Hermes (god of science, etc.) as the author of occult science and esp. alchemy; see -IC.
So hermetically XVII.

Hermetic of or relating to an ancient occult tradition encompassing alchemy, astrology, and theosophy; the word comes in the mid 17th century from the name of Hermes, identified with Thoth, regarded as the founder of alchemy and astrology.
hermetic seal an airtight seal (originally as used by alchemists).