Many people mistakenly thought that inflammable meant “resistant to burning,” which it doesn’t.

The -in Prefix Has Multiple Meanings

The in- prefix usually has a negative meaning, as in indelible which means “can’t be deleted” and indefinite, which means “not defined.”

But the in- prefix has another use too: It can act as an intensifier. That’s what it’s doing in words such as intoxicate and indent—it’s just intensifying toxicate and dent—and that’s what it’s doing in inflammable. It’s just intensifying the meaning of the flammable part.

‘Flammable’ Is About Safety

According to the Merriam-Webster Dictionary of English Usage, the National Fire Protection Association felt that people thinking inflammable things wouldn’t burn was enough of a safety problem that they starting using the word flammable instead. And that’s why flammable is so popular today. In the 1920s, almost nobody used the word flammable, but by the 1970s, it had overtaken inflammable.

That’s your Quick and Dirty Tip: flammable and inflammable mean the same thing, and they're both proper words, but for safety reasons, flammable is the better choice.

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