The words “flammable” and “inflammable” mean the same thing: “easily set on fire.” Why is this?
In English, the in- prefix is often used to reverse the meaning of an adjective. Thus inactive is the opposite of active and inelegant is the opposite of elegant. So why isn’t inflammable the opposite of flammable?
The reason is that the in of inflammable is not the prefix meaning “not.” Inflammable derives from the Latin in meaning into and flamma, “a flame.” Flammable derives simply from flamma. Inflammable is thus very close to the word enflame, which has the same origin.
In practice, it can be confusing to have two words that sound as if they could be opposites but which actually mean the same thing. It could even be dangerous, if “inflammable” were taken to mean “not flammable.” The Compact Oxford English Dictionary recognizes this and recommends using “flammable” at all times :
The words flammable and inflammable have the same meaning. It is, however, safer to use flammable to avoid ambiguity, as the in- prefix of inflammable can give the impression that the word means ‘non-flammable.’
As this quotation makes clear, the opposite of flammable is not inflammable but “non-flammable” or simply “not flammable.”