helium-filled

The word is a modern English compound adjective, transparently combining the name of a chemical element with a common participial adjective. 'Helium' itself has a celestial origin, coined from the Greek word for the sun ('hēlios') after the element was first detected in solar spectra. The second element, 'filled', descends from the robust Germanic verb 'to fill', related to the concept of fullness. When combined, the term follows a straightforward descriptive logic common in technical English: 'X-filled' denotes an object whose interior volume has been occupied by substance X, in this case, the inert, lighter-than-air gas helium.

Structure: helium (chemical element) + filled (made full) - helium: From New Latin 'helium', from Greek 'hēlios' (sun) + the New Latin suffix '-ium' (denoting a metallic element). In this compound, it functions as a noun specifying the substance. - filled: From Old English 'fyllan' (to fill), from Proto-Germanic 'fullijaną'. In this compound, it functions as a past participle adjective meaning 'made full of'.