Usage Note: Inflammable means "combustible," and has the same meaning as the word flammable. How is this possible? The prefix in- here is not the Latin negative prefix in- (which is related to the English un- and appears in words such as indecent and inglorious) but is derived from the Latin preposition in, "in." This prefix also appears in the word inflame. However, some people mistakenly think that inflammable means "not flammable." Therefore, for clarity's sake, it is safest to avoid inflammable altogether and use flammable instead.