English has 2 prefixes that look alike. They are both “in”. One “in” prefix is negative, the other means within or on.

For me, the first usage that I think of is the negative. That is, words like inaccurate, inappropriate, or inconceivable.

The other prefix that means within or on shows up in words like inform, invite, inhabit, invest, or inscribe.

When I first heard of this, I assumed that this was because the two prefixes came from different roots. I was soon disabused of this. English got both prefixes from Latin which had the same issue.

All of which is a roundabout way of getting to flammable and its buddy inflammable. Here we go:
Flammable means that something burns.
Inflammable means that something really, really burns. I mean, boy, is that flammable.

Apparently, this is because the “in” in “inflammable” is the “within” type of in and not the “negative” in. Think of how “Inflaming the masses” doesn’t mean calming them down.

It’s easy to imagine folks thinking that inflammable materials wouldn’t burn. It’s also easy to imagine folks tossing very volatile chemicals labeled “inflammable” onto fires in an effort to put fires out.

Sometime in the 70s or whenever, various fire departments got tired of putting out fires that were caused by this confusion by tossing out “inflammable” entirely only using “flammable” and “nonflammable”.

Pam and I both remember being taught in school as kids that “IMflammable” meant “not flammable”. The internet says that didn’t happen and that “IMflammable” is a spelling mistake.

Whatever. The “flammable”/”nonflammable” thing still makes more sense.