|This salamander is inflammable. He finds the fire perfectly habitable.|
flammable/inflammable - Inflammable is the older word in English and comes directly from Latin, where the in- prefix in this case actually means “in” (as in “in flames”) rather than “not.” I don’t know why people started using flammable, some two centuries later, but probably just because inflammable sounds so much like it ought to mean not flammable. For another century the older inflammable was more common in the US, and the newfangled flammable more common in Britain. It was during World War II that flammable really became widespread on both sides of the Atlantic. When the USA joined the war, the Allied leaders asked them to label explosives with “Flammable” so as not to cause confusion - and explosions.
|This one is flammable, and he finds the fire inhabitable.|
habitable/inhabitable - Here’s another case of quirky etymology. When you can live somewhere, it ought to be habitable, and indeed it was, since the late fourteenth century. At the same time, inhabitable meant, of course, “not habitable” with the in- prefix meaning “not.” Nowadays inhabitable means the same as habitable, so how did it change to its opposite? Simple - it isn’t really the same word at all. The original inhabitable has become obsolete, while a new word was coined. Taking the word inhabit meaning “to dwell in” (in which the in- prefix means “in”), people added the -able suffix around 1600 to get, perfectly logically, inhabitable meaning “able to be dwelt in.”
[Pictures: Salamander (the device of Francis I), woodcut perhaps by Christopher Plantin, from Devises Heroïques by Claude Paradin, 1557 (Image from Glasgow University);
Salamander (printer’s device), woodcut from a book printed by Charles Pesnot, 1567.]