The word 'thermos' is a proprietary name that became generic. It derives directly from the Greek adjective 'thermos', meaning 'hot'. The term was adopted as a trademark in 1907 for a vacuum flask invented by Sir James Dewar, leveraging the Greek root to suggest its function of retaining heat. Over time, the brand name lost its trademark protection and evolved into a common noun in English, describing any vacuum-insulated container. Its journey from a Greek descriptor to a specific brand and finally to a general term exemplifies the process of genericization.

Structure: therm (heat) + os (noun suffix) 'therm' originates from Greek 'thermos' (hot, warm). 'os' is a common noun-forming suffix in English, often used in scientific or brand names.