the Emerald Tablet of Thoth

A hermetic seal is one that is air-tight, or in other words, tightly closed. Hermetic seems to be a reference to Hermes, the ancient Greek messenger-god. But how did Hermes become associated with an air-tight seal?

In the third century A.D., the philosophical school of the Neo-platonists arose in Alexandria. They associated the Egyptian god Thoth, the god of alchemy and mystical secrets and inventor of a magical seal, with Hermes, calling him Hermes Trismegistus. In the seventeenth century, English writers began using the adjective hermetic to refer to things that were sealed or secret. From Jeremy Taylor’s 1663 Sermon preached at the funeral of John (Bramhall) late Lord Archbishop of Armagh:

Not nature, but grace and glory, with an hermetic seal, give us a new signature.

So Hermes is associated with the seal only through the habit of conquering nations to associate their gods with the local gods.

(Source: Oxford English Dictionary, 2nd Edition)