Why Inflammable Is Not the Opposite of Flammable

Combustible and incombustible are opposites but flammable and inflammable are synonyms. Why? The in- of incombustible is a common prefix meaning "not," but the in- of inflammable is a different prefix. Inflammable comes from Latin inflammare ("to inflame"), itself from in- (here meaning "in" or "into") plus flammare ("to flame"). Flammable also comes from flammare. In the early 20th century, firefighters worried that people might think inflammable meant "not able to catch fire," so they adopted flammable and nonflammable as official safety labels and encouraged their use to prevent confusion. In general use, flammable is now the preferred term for describing things that can catch fire, but inflammable is still occasionally used with that meaning as well.