The Etymology of Fuel
Fuel and focus look unrelated, but they descend from the same Latin word: focus, the hearth of a Roman house. In Vulgar Latin a derivative *focalia named the firewood that tenants owed their lord; this entered Old French as fouaille and then English around 1300 as feuel, fewell, or fewel — meaning, narrowly, firewood. For four centuries that was its only sense. The Industrial Revolution forced an expansion: coal, then oil, then petroleum and gasoline all wanted the same name, and fuel was abstracted from wood to mean any combustible substance. Fuel cell appears in 1922; fossil fuel in 1838 (much earlier than most expect, originally meaning coal); the metaphorical to fuel a fire dates to the 1590s; to fuel debate or fuel speculation are modern, mostly post-1900. Meanwhile focus, the same word at root, took an astronomical turn through Kepler's Latin in 1604.