Hermetic= being airtight and impervious
Hermes Trismegistus was believed to have invented a magic seal to keep vessels airtight and from his name, the adjective hermetic, was derived. It is a system which explains causes of diseases and operations of medicine. Hermes was a greek mythological character identified by the Greeks with the Egyptian god Troth. To him was ascribed authorship of various works on astrology, magic, alchemy, and medicine (remember: myth is not truth).