Inflammable means able to be put “in flames”. But most people read “in-“ in this case as a negation, and therefore assume it’s the opposite. So “flammable” is

Inflammable means able to be put “in flames”. But most people read “in-“ in this case as a negation, and therefore assume it’s the opposite. So “flammable” is born and people start using it and thinking the original form is opposite. But inflammable materials are dangerous to treat otherwise! Since no one confuses “flammable” for inert, for safety reasons, the word “flammable” is used when labeling inflammable material. The Simpsons made a joke about it once with Dr. Nik.