Related to or characteristic of Hermeticism, or so tightly sealed that nothing can pass through.
From Hermes Trismegistus (the legendary founder of Hermetic philosophy) via Medieval Latin hermeticus. The sense of 'airtight sealing' comes from Hermes Trismegistus's association with alchemy and sealed vessels.
The term 'hermetically sealed' is used in kitchens and labs today, but people usually don't realize they're invoking an ancient Egyptian god of wisdom every time they seal a container!
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