The word flammable, meaning easy to set on fire, derives from the word inflammable, meaning… easy to set on fire. The reason flammable was created was because people thought inflammable meant not-flame-able.
But where did inflammable come from?
My guess would be that it derives from the word inflame, meaning… to set on fire. This use of the in- prefix is similar to inflate and increase. The reason inflate and increase didn’t get a similar treatment is probably because flate is far enough from float to be considered different words, and we associate crease with a different meaning (to fold neatly) then we did back then (
probably “divide” or “make greater”. Apparently, crease actually derives from the Latin word crea, meaning to create.)