For example, “flammable” and “inflammable” both describe an object that can easily catch on fire. I can also think of “ceased” and “deceased”, both of which can mean someone or something has been brought to an end.
edit: Some people are including words that can also mean its opposite (like sanction or table), those are cool too! The more weird words, the better!
I’m not sure this is correct, but I’ve heard that flammable and inflammable being used as synonyms is recent. Originally, inflammable meant able to burst into flames without a significant ignition source. Like a pile of oily rags or something that could catch fire because it was left out in the sun or just got too warm.
It sounds like it should mean something flame-proof. Like destructible - indestructible.
Yeah, adding in- as a prefix often does mean that, but it can also mean in, on, or into (among others) as a prefix or just part of the core word. And in this case inflammable comes from adding the suffix -able to inflame, a word that already starts with in- in it’s into meaning. And one definition of inflame is “to burst into flames.”