gas: [17] We get gas from a Flemish pronunciation of Greek kháos ‘chasm, void’ (a derivative of Indo-European *ghəw- ‘hollow’, and source of English chaos [15]). The Flemish chemist J B van Helmont (1577–1644) used the Greek word to denote an occult principal, supposedly an ultra-refined form of water, which he postulated as existing in all matter.
The sound of Greek kh is roughly equivalent of that represented by Dutch and Flemish g, and so the word came to be spelled gas. Its modern application to any indefinitely expanding substance dates from the late 18th century. The derivative gasoline, source of American English gas ‘petrol’, dates from the late 19th century. => chaos[gas etymology, gas origin, 英语词源]