dear Sanjeev,
Historically, flammable and inflammable mean the same thing. 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.
HTH.
-Vikas
