[omneity]

Not quite. "in" here as a prefix is not a negation thing but to _do_ something like "en" in "enhance" or "encapsulate". The word's actual latin root is "inflammare" which means to put something _in_ flames. The subject is the one doing the burning and it's transitive.

Flammable on the hand comes from "flammare", which means for something to catch fire, and is intransitive instead, i.e. the subject is the one catching fire.

The actual opposite of inflammable is uninflammable, which I reckon is only in British English at this point and mostly lost in American English.

[forty]

In French we don't have flammable, only _inflammable_ (meaning that it CAN catch fire). And the opposite is _ininflammable_ ^^

Something in flames is "enflammé" (there is the en- prefix ^^).

[gessha]

As I’ve followed the news for many years now, not many things in France are inflammable :D