2. Easily ignited and capable of burning rapidly.
The word inflammable actually means "able to burn"; while "nonflammable" means "unable or unlikely to burn".
The prefix "in-", used with inflammable is an intensifier not a "negative" as with such words as "inhospitable", "inhuman", "insensible", "indecent", "inadequate", etc. It has been a mistake believed by many people that inflammable meant "not flammable" and some serious injuries resulted over the years because of this misunderstanding.
As a consequence, many English-speaking countries passed laws that have required manufacturers of materials and substances that burn easily to use the word "flammable" in place of inflammable, which is considered less ambiguous or confusing.
So remember that inflammable comes from "inflame", which, in this case, means "to set on fire" and not from "in-", "not" + "flammable", "combustible".
In the confusion at the store, which was caused by a customer's inflammable temper, Joseph was unable to read the sign to see if the can of solvent was flammable or non-flammable.