Inflammable and flammable both mean “combustible.” Inflammable is the older by about 200 years. Flammable now has certain technical uses, particularly as a warning on vehicles carrying combustible materials, because of a belief that some might interpret the intensive prefix in- of inflammable as a negative prefix and thus think the word means “noncombustible.” Inflammable is the word more usually used in nontechnical and figurative contexts: The speaker ignited the inflammable emotions of the crowd.
early 15c., in medicine, "liable to inflammation," from Middle French inflammable and directly from Medieval Latin inflammabilis, from Latin inflammare (see inflame). As "able to be set alight," c.1600. Related: Inflammability.