Many words with the prefix “in-” mean the opposite of the same word without the prefix.
Such is NOT the case with inflammable. Inflammable means “easily inflamed,” which is basically the same thing as flammable, not the opposite. Rather than the usual meaning of “not,” the Latin prefix “in-” is used as an intensifier with the word “flammable.” (I would explain why, but I took French in high school instead of Latin, much to Mr. Borelli’s dismay. Why would I want to learn a language that nobody speaks?)
According to Jack Lynch, author of The English Language: A User’s Guide, the word inflammable long predates flammable (1605 vs. 1813). The word flammability appeared in the seventeenth century, but it then disappeared until the twentieth century. In the twentieth century, flammable was increasingly used to mean “able to be set on fire,” while inflammable has been losing ground.
Now, the problem occurs when people think the “in-” prefix means “not” flammable, as in the examples above. Usually in such situations, they don’t have time to look up the meaning of the word in the dictionary to discover that it really means “flammable.” As Jack Lynch puts it: “Someone trying to put out a fire who sees a bucket of something labeled INFLAMMABLE has good reason to hope for perfect clarity.”
Therefore, if there’s any chance at all that somebody could misunderstand your written instructions and go up in flames as a result, use “flammable” or “nonflammable” to be perfectly clear. And if you’re the one trying to put out a fire, whatever you do, don’t use the liquid labeled “inflammable”!