A common misconception is that because the word “inflammable” allegedly contains the prefix “in-,” it means the object it describes exhibits resistance to fire. Indeed, the infamous non-word “flammable” was invented primarily as a result of this unfortunately widespread impression, and one rarely sees “inflammable” used at all anymore.
Inflammable’s origins can be found in the Latin “inflammare,” meaning “to set aflame,” with in- not being a prefix at all but rather a remnant of the Latin form.
As usual, Strunk and White (authors of the excellent “The Elements of Style”) put it best: Unless you are “concerned with the safety of children and illiterates, use ‘inflammable.'”