Easily ignited and capable of burning rapidly.
[From Latin flammāre, to set fire to, from flamma, flame; see bhel-1 in the Appendix of Indo-European roots.]
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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.