Flammable/inflammable

Meaning- Easily ignited and capable of burning rapidly. Inflammable means “combustible,” and has the same meaning as the word flammable

“Caution: this material is flammable.”

Flammable and inflammable make my list because they make no sense. What is the point of having two words which sound like they should be the opposite of each other but in fact mean precisely the same thing? It’s bizarre. And is there a word meaning not flammable? No! You have to say “not flammable”. I am considerably unimpressed.