'in' negates it? What about inflammable..?,(reply>), yeah it's strange. Like words have multiple meanings, some prefix has multiple meanings too. Heck "able" has multiple meanings (X-able can mean "can X", "can be X'd", "deserves to (be) X", "tends to X", etc. Examples include variable, debatable, honorable, comfortable for each meanings respectively,(reply>), It's strange if true. Why not 'enflame' like engulf? That would be more logical, or less ambiguous. Especially as it is French origin (en meaning in). It's like someone said, this is a name for something dangerous, let's make it easy to misunderstand.,(reply>), In as inside from the word inflame not negation in that case.