"in" denotes the opposite, so "inflammable" has been used to mean not flammable due to expectations of grammatical consistency, and as a result the word is generally preferred to be avoided entirely nowadays in favor of either "flammable" (or "highly flammable"), or "nonflammable".
This interpretation is based on incorrect decomposition of the word.
In this case, the "in" prefix means "in/on". Think of it as "inflame" + "able". Similar to how "inflammation" doesn't mean "a state of not burning" and "inflamed" doesn't mean "not burning". Also see "ingress", "ingest", "inaugurate".
I'm only an armchair etymologist and this is wild speculation, but I think that the meaning of the "in" prefix might depend on whether we get the word directly from Latin, or whether it comes through French.
> I'm only an armchair etymologist and this is wild speculation, but I think that the meaning of the "in" prefix might depend on whether we get the word directly from Latin, or whether it comes through French.
French has both meanings: the negation as in interdit (forbidden) or impossible (well, impossible); or “in”, “towards”, “change” as in intérieur (interior), inflexion (inflection), or indeed inflammable (from the Latin inflammabilis).
Both meanings also exist in Latin.
What I found fascinating learning English is “inhabit”, which also sounds like the opposite of its actual meaning. Is obviously the second meaning, but then the prefix is redundant because it came from the Latin habitare, which is the verb with the same meaning.
"Flammable" is such a weird word. Folk etymology would derive it from a transitive verb "to flame" that doesn't really exist (i.e. is not used, at least not that way).
There is a transitive verb "to inflame", which is common. It derives from the noun "flame" and the prefix "in-", which when applied to nouns makes it a verb meaning "to cause [the noun]".
They also ignored the common word "inflammation", which nobody thinks means "to stop your tissues from flaring up".
None of that matters. People parsed "inflammable" differently and arrived at a new meaning. But I just find it odd that, while doing that parsing, they never considered that they never use the verb "to flame" in ordinary speech.
'Flame' as in 'to catch fire' has some rare usage in English- "The main blaze of it is past, but a small thing would make it flame again" says Shakespeare. The more common usages are metaphorical- 'flame with passion', or more modern 'flamed them online', though I don't really see that usage much anymore either.
