I sometimes hear “inflammable” when people mean “nonflammable”.

Problem:
The adjective “inflammable” is synonymous with the adjective “flammable”, not the adjective “nonflammable”.

Explanation:
There is a funny scene in an episode of “The Simpsons” in which Apu Nahasapeemapetilon, the cartoon character whose first language is NOT English, says, “Flammable. Inflammable. I don’t understand this language.”

The primary definition of the adjective “flammable” is combustible.

The primary definition of the adjective “inflammable” is combustible.

The primary definition of the adjective “nonflammable” is not combustible.

In other words, “inflammable” is synonymous with “flammable”. The typical distinction in usage of these two adjectives is that “flammable” is usually applied to physical things (e.g., “flammable gas tank”) whereas “inflammable” is usually applied figuratively (e.g., “inflammable feelings of the dissatisfied constituents”).

The misuse of “inflammable” as a synonym for “nonflammable” apparently comes from the mistaken belief that the prefix “in” in “inflammable” means not. The correct meaning of the prefix “in” in “inflammable” is in.

A simple way to remember that “inflammable” means combustible instead of not combustible is to remember the verb from which “inflammable” is constructed: “inflame”.

Solution:
Use “nonflammable” when you’re talking about something that is not combustible. Use “flammable” or “inflammable” when you’re talking about something that is combustible. For more refined usage, use “flammable” for literal things (such as cars or boats) and “inflammable” for figurative things (such as emotions).