A reader recently asked me to explain why inflammable means “flammable” rather than “not flammable.” She wasn’t confused by which term to choose, but she wasn’t sure why things were the way they were.
Most copyeditors easily recognized both flammable and inflammable as meaning “able to set on fire.” The second term also has a figurative sense, referring to the ability of our thoughts or emotions to become inspired or aroused. We know that if we want to say something is not able to be set on fire, we need to use nonflammable or, less commonly, non-inflammable.
The reason for inflammable’s meaning is actually quite simple: the prefix in- is different from what we first think of. In many words, in- is the Latin prefix meaning “not,” as with inaccurate, inappropriate, and invalid. And that’s what we commonly think of with inflammable.
But inflammable’s prefix is from the Latin preposition in, as with influence, inscribe, and instruct. The story of how we got here is less simple.
Inflammable entered English first, around 1425, as a medical term meaning “liable to inflammation,” according to the Chambers Dictionary of Etymology. The term was borrowed from Middle French and taken directly from the Middle Latin inflammabilis, says the Online Etymology Dictionary. Shakespeare’s 2 Henry IV (1597) is the first documented usage of inflammation to refer to one’s emotional state in, setting up our figurative usage of inflammable:
Falstaff: They are generally fools and cowards—which some of us would be
too but for inflammation.
Inflammable picked up its “able to set on fire” meaning in 1605. Once it was used this way, the confusion started. Although in- has more than one meaning, many people gravitated to the “not” meaning. Had enough people done this, inflammable would have picked up a new meaning; but clearly not enough people did.
Filling the gap
Maybe that’s why, in 1813, flammable appeared. After two hundred years of conversations like this, language users finally found a solution:
Tom: Watch your skirts near the hearth, my dear. That material is quite inflammable. Cousin Beth is still recovering from the burns she received last month.
Mary (pushing the soup pot over the fire): But you say my skirts are inflammable, darling. I shouldn’t have to worry … Oh my!
But with two words meaning “able to be set on fire,” there was still a gap: how to say “not able to be set on fire.” In relatively short order, at least as far as language change goes, non-inflammable and nonflammable appeared on the scene:
With certain heated mixtures of gases, where the non-supporting or non-inflammable elastic fluids are in great quantities, combination with oxygen will take place. —Philosophical Transactions of the Royal Society of London (1817)
A number of visitors witnessed another practical demonstration of the enormous power to resist fire possessed by ‘non-flammable’ wood in comparison with ordinary timber. —Science (August 19, 1898)
In the 1920s, the United States’s National Fire Protection Association adopted flammable to avoid the potentially harmful confusion with inflammable. Since then, flammable has been more common in American English, while inflammable has remained more common in British English, although inflammable is used in its figurative sense in American English often enough.
The trick to using either term is to be sure the context is clear. And if you need to say “not able to be set on fire” in just a single word, nonflammable is your best bet. Non-inflammable remains a rarity.