If you want to keep cookies crisp for a long time, store them in a jar with a hermetic, or airtight, seal. Hermetic means sealed so that no air can get in.
The word can be used metaphorically as well. A child who is completely protected from the outside world might be said to come from a hermetic environment. The word comes from the name of the Greek god, Hermes Trismegistus, who was a magician and alchemist and was credited with creating the process for making a completely airtight glass tube, a god-like feat if there ever was one.