English is a trickster of a language, evidenced by the fact that two words that appear to be antonyms can actually mean the exact same thing. However, every once in a while we come across a pair of words that it really would be better to not confuse. A fine example of this is flammable and inflammable.

Why are these two words so confusing? Well, flammable and inflammable both mean “able to be set on fire.” Strangely, the in- prefix in inflammable doesn’t make the word mean “not flammable,” as you might think.

Why do flammable and inflammable mean the same thing?

There is a fairly clear reason for why both these words carry the same meaning. The prefix in– can have a variety of meanings:

In- can have the force of the English preposition in, such as in inland or inward.

In- can have the force of the Latin preposition in, also meaning “in,” such as intonation or infatuation. The Latin commonly added it to verbs.

In– can also serve to negate, from another Latin in– and corresponding to English un-, as seen in words like inexpensive (“not expensive”) or impossible (not possible, where the –n– becomes an –m– for ease of articulation).

On the model of words like inexpensive/expensive, we might reasonably think inflammable is “not flammable.” But alas, the in- here is of the second variety highlighted above.Inflammable comes from the Latin inflammāre, “to inflame.” Think about this as “setting on fire,” of “putting flame into or onto (something).” Its root flammāre, which also means “to set on fire.” Tack on the –able suffix and you get flammable.

The English word flame is indeed related, via Latin flamma (“flame, fire”).

Which word is older?

Of the two, inflammable is older. It’s recorded by the 1600s, used in early scientific texts and often of hydrogen gas, which is extremely flammable. (Or inflammable, if you prefer.)Flammable is found by the 1800s. It was apparently an English-language coinage … and inflaming our patience and commonsense ever since.

When did flammable come into common use?

It’s reported that, as early as the 1920s, the eagle-eyed language guardians of the National Fire Protection Association (NFPA) realized that many people were viewing the in– in inflammable as a negative prefix and were at risk of consequently incinerating themselves at a much higher rate than was desirable. (Is any rate really desirable, though?)

The NFPA advocated to have flammable used exclusively for warning labels (such as are found on mattresses, oil cans, and other things that will catch on fire if you put a match to them) and managed to nudge our language toward a more sensible path.

According to Google Ngrams, flammable takes over inflammable in frequency in the 1970s, likely corresponding to such safety efforts. Flammable (along with combustible) is now more common in technical contexts, particularly as a warning on vehicles carrying combustible materials. Inflammable is the word more usually used in nontechnical and figurative settings (e.g., The speaker ignited the inflammable emotions of the crowd).

How can we refer to something that isn’t flammable, then?

In a word? Nonflammable. Now that’s makes sense.