Inflammable came first.
It is odd that these seemingly opposite words have the exact same meaning. One of them came before the other, which makes us wonder why the second was needed at all.
Let’s see what Merriam Webster has to say.
Inflammable and flammable are synonyms and mean “able to burn” even though they look like opposites. In this case, rather than the prefix in- meaning “not,” as it often does, “inflammable” comes from the latin verb inflammare, which means “to cause to catch fire.” “Flammable” was coined later from a translation of the latin verb flammare (“to catch fire”), which inflammare is related to.
In Latin, the prefix in means “to cause”, while in English it means “not”. This leads to the apparent contradiction inherent in the word inflammable.
The English word inflammable came into use in the 17th century. Merriam Webster further explains that, in the early 19th century “a scholar translating a Latin text coined the English word flammable from the Latin flammare“.
In 1920 the National Fire Protection Association (NFPA) apparently “realized that many people were viewing the in– in inflammable as a negative prefix“. The NFPA encouraged the use of the word flammable over inflammable on warning labels to avoid confusion and the resulting danger that would cause.
Beginning in 1920, the use of the word flammable rose, while inflammable declined.
The chart above comes from the Google NGram viewer.
It’s interesting to note that the use of the word inflammable in 1800 was about 5 times greater than both inflammable and flammable in 2000. In fact, it peaked in 1787 at about 20 times greater than current use.