What to Know
The Latin words inflammare (“to cause to catch fire”) and flammare (“to catch fire”) came into English at different times to become the synonyms inflammable and flammable. Although it seems logical that inflammable might mean “not flammable,” the word actually means the opposite, because the in- prefix of inflammable comes from the Latin word meaning “in” or “into.” Using nonflammable is one way to ensure clarity.
"When cooking over a gas stove, avoid wearing loose, (flammable/inflammable) clothing that could catch fire easily." Which word is correct: flammable or inflammable?
Trick question: both flammable and inflammable are correct, as they both mean "capable of being easily ignited and of burning quickly." This makes no sense to the Modern English speaker. In English, we think of in- as a prefix that means "not": inactive means "not active," inconclusive means "not conclusive," inconsiderate means "not considerate." Therefore, inflammable should mean "not flammable."
The Latin Inflammare
That would make sense—if inflammable had started out as an English word. We get inflammable from the Latin verb inflammare, which combines flammare ("to catch fire") with a Latin prefix in-, which means "to cause to." This in- shows up occasionally in English words, though we only tend to notice it when the in- word is placed next to its root word for comparison: impassive and passive, irradiated and radiated, inflame and flame. Inflammable came into English in the early 1600s.
Things were fine until 1813, when a scholar translating a Latin text coined the English word flammable from the Latin flammare, and now we had a problem: two words that look like antonyms but are actually synonyms. There has been confusion between the two words ever since.
The True Opposite of Inflammable
What do you do? To avoid confusion, choose flammable when you are referring to something that catches fire and burns easily, and use the relatively recent nonflammable when referring to something that doesn't catch fire and burn easily. Our files indicate that use of flammable and nonflammable has increased in print over the last few decades, while use of inflammable has decreased.