meaning - Is it inflammable or flammable?

You sometimes see on tank trucks or petrol tankers carrying gas or oil or other liquids that catch fire quickly the word inflammable painted in bold red on them.Should not the correct word be flammable?

Answer

They are two different terms with different, though related, etymology. Flammable is the more recent and also the more common of the two: (Etymonline)

inflammable

• 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.

flammable:

• 1813, from stem of Latin flammare "to set on fire" (from flamma; see flame (n.)) + -able. In modern (20c.) use, a way to distinguish from the ambiguity of inflammable.

Ngram inflammable vs flammable

from The grammarist:

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  There is no difference in meaning between flammable and inflammable. Both describe things that are capable of burning or easy to ignite, but in all modern varieties of English, flammable is preferred.

  
  


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  Inflammable, derived from the verb inflame, is the original word. But because the first syllable is easily misinterpreted as the common negative prefix in- (as in, for example, inescapable, invulnerable, inorganic), the word has always caused confusion. Because this confusion can have dangerous real-world consequences, the shift from inflammable to flammable is welcome.

  
  


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  The less confusing flammable did not enter common use until the early 20th century, but it quickly became the prevalent spelling. Inflammable is still common on product labels and appears from time to time in edited publications, but it fell out of favor around 1970.