This is something that hopefully you’re already aware of. If not, let me perform a public service by informing you of the following:
Flammable and inflammable both mean the same thing. And just to make it absolutely clear, something described as flammable/inflammable can burn easily. This has long been held up as an example of the confusing nature of English, but it’s also particularly noteworthy in that it could have a serious practical effect on someone’s life. How easily might someone assume that something inflammable is safe to use around fire, and end up setting it alight? Why then, when the word is so obviously confusing, do we have it at all? Or, why does it not mean can’t be set alight?
First of all, let’s look quickly at why exactly inflammable is a confusing word. In- is often used as a prefix with adjectives and nouns to indicate the opposite of the word it precedes, e.g. inaction, indecisive, inexpensive etc. It’s logical enough then to assume that the word inflammable is the opposite of flammable, especially considering that there do exist substances which cannot be set alight. Not only that, but if you think about it, they’re also quite likely to be found near flammable substances. If you’re working with things which can catch fire quite easily, you’re going to want to have some materials at hand which can’t, to protect yourself in case of an accident. There seems to be no logic at all to the existence of inflammable.
But that’s only the case when looking at the situation from a modern perspective, because inflammable actually pre-dates flammable. The word dates back to at least the early 17th century, and is derived from the Latin verb inflammare, meaning to set on fire. This verb is also the origin of the modern words to inflame/inflammation, meaning to swell or to provoke angry feelings (the link between setting something on fire and rousing strong feelings in someone is logical enough). So back when Latin was a much more prominent part of daily life, inflammable made sense. However, as time went on, and Latin receded, while words with in- as a negative prefix became a bigger part of our daily lexicon, confusion began to creep in. By the early 19th century, flammable had entered into common usage, and by the 20th century had become pretty widespread, as it lacked the ambiguity of inflammable. In the 1920s, the National Fire Protection Association in the United States began to publicly urge people to use flammable, to avoid accidentally starting fires.
And because of such moves, and how naturally logical it seems to most of us to use flammable, it’s by far the more common word now. Still, you may end up coming across some old canister or other marked as inflammable, so remember what it means. Oh, and of course the actual opposite of flammable is non-flammable. No confusion there at least!