[This is a fast-access FAQ excerpt.]
People often ask why "flammable" and "inflammable" mean the
same thing. The English words come from separate Latin words:
inflammare and the rarer flammare, which both meant "to
set on fire". Latin had two prefixes in-, one of which
meant "not"; the other, meaning "in", "into", or "upon", was the
one used in inflammare. "Inflammable" dates in English from
"Flammable" is first attested in an 1813 translation from Latin
It was rare until the 1920s when the U.S. National Fire Protection
Association adopted "flammable" because of concern that the "in-" in
"inflammable" might be misconstrued as a negative prefix.
Underwriters and others interested in fire safety followed suit.
Benjamin Whorf (1897-1941), the linguist who shares credit for the
Sapir-Whorf Hypothesis that language shapes thought, may have been
influential in promoting this change. Merriam-Webster Editorial
Department writes: "Though we have been unable to confirm that
Benjamin Whorf was responsible for the word's adoption, the theory
seems plausible enough: he was, in fact, employed by the Hartford
Fire Insurance Company from 1918 to 1940, and was widely recognized
for his work in fire prevention."
"Flammable" is still commoner in the U.S. than in the U.K.;
in figurative uses, "inflammable" prevails (e.g., "inflammable
Other words where an apparently negative prefix has little
effect on the meaning are: "to (dis)annul", "to (de)bone", "to
(un)bare", "to (un)loose", and "to (un)ravel". "Irregardless"
(which probably arose as a blend of "irrespective" and "regardless";
it was first recorded in western Indiana in 1912), means the same as
"regardless", but is not considered acceptable.