E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Closed securely. Thus we say, My lips are hermetically sealed, meaning so as not t utter a word of what has been imparted. The French say close-fitting doors and windows shut hermetically. When chemists want to preserve anything from the air, they heat the neck of the vessel till it is soft, and then twist it till the aperture is closed up. This is called sealing the vessel hermetically, or like a chemist. (From Hermës, called Trismegistus, or thrice-great, the supposed inventor of chemistry.)