text: flammable vs. inflammable? why do we say things that can catch on fire are inflammable?  in means "not", for example indescribable, indecent, injustice, inappropriate, etc.  remove the in from the examples and you have the opposite meaning. inflammable means "can be inflamed". the "in" does not mean "not" in this case.  it's one of those exceptions like not pronouncing the "k" in knife.