Comes the 19th century, and some well-meaning soul dreamt up the word “flammable,” basing it on a slightly different Latin word, “flammare,” meaning “to set on fire.” There was nothing terribly wrong with “flammable,” but it never really caught on. After all, we already had “inflammable,” so “flammable” pretty much died out in the 1800’s.

“But wait,” you say, “I saw ‘flammable’ just the other day.” Indeed you did. “Flammable” came back, one of the few successful instances of social engineering of language.

The Latin prefix “in,” while it sometimes means just “in” (as in “inflammable”), more often turns up in English words meaning “not” (as in “invisible” — “not visible”). After World War Two, safety officials on both sides of the Atlantic decided that folks were too likely to see “inflammable” and decide that the word meant “fireproof,” so various agencies set about encouraging the revival of “flammable” as a substitute. The campaign seems to have worked, and “inflammable” has all but disappeared.

That left what to call something that was not likely to burst into flames, but here the process of linguistic renovation was easier. “Non-flammable” is a nice, comforting word, and besides, it’s far easier on the tongue than its now thankfully obsolete precursor, “non-inflammable.”

The Oxford English Dictionary adds this usage note: Historically, flammable and inflammable mean the same thing. However, the presence of the prefix in- has misled many people into assuming that inflammable means “not flammable” or “noncombustible.” The prefix -in in inflammable is not, however, the Latin negative prefix -in, which is related to the English -un and appears in such words as indecent and inglorious. Rather, this -in is an intensive prefix derived from the Latin preposition in. This prefix also appears in the word enflame. But many people are not aware of this derivation, and for clarity’s sake it is advisable to use only flammable to give warnings.

Flammable – Matter catches fire when set it to
Inflammable – Matter catches fire without a source of ignition. for example exxessive presssure, sudden impact, react with surrounding environment.
And Thank God, there is no word called Non-inflammable, it’s just non-flammable

2nd question—- figurative context means not in literal sense…

USAGE Flammable and inflammable are interchangeable when used of the properties of materials. Flammable is, however, often preferred for warning labels as there is less likelihood of misunderstanding (inflammable being sometimes taken to mean not flammable). Inflammable is preferred in figurative contexts: this could prove to be an inflammable situation.