I've been listening to Alanis Morissette, so I started thinking about why there is such a fuss about how she used the word "ironic" incorrectly. In my experience, literary irony is generally just as well described by the term "sarcasm", but there is no one word that really describes the "unfortunate coincidence" meaning that "irony" has in the song.
So now I'm wondering why it's such a crime to use this word in a way that is colloquially understood, even if it's not the meaning the word has had in the past. English is an ever-changing language, and we have plenty of examples of word meanings getting garbled, so I'm wondering why so many people rebel against this specific change.
Thus my question: What does it take for a colloquial meaning of an English word to become canon, and what will it take for people to get over the fact that "irony" has a new colloquial use?