G. Randolph Mayes in The Dance of Reason:
If I could take one word back from the English language, change its common meaning without anyone noticing, it would be: rationalize.
Don't get me wrong. I think its cool that English words evolve over time, even when it's due to error. (Hey, that's how evolution works, right?) I don't mind that it's now OK to say literally when you mean figuratively or nonplussed when you mean unperturbed. I don't even really care about the plundering of philosophical terms like begs the question, which now means something completely different in the vernacular (raises the question) than it does when we use it in informal logic (takes for granted the point at issue).
But rationalize? Come on. Certainly the obvious and intuitive meaning of rationalize is: to make more rational. But today the term has come to mean almost exclusively the opposite: to make something appear rational, when it is not. Dude, you know this is bullshit, you're just rationalizing.
Well, before I explain why I really find this meaning irritating, I have to admit that it isn't quite as perverse as I make out. The suffix 'ize' means 'to cause to be or be like'. And, of course, once you dance into the semantic cloud of similarity and appearance, it is a small step from 'be like' to 'seem like.' Still, while the word rational isn't the only term to suffer izing in this way (moralize, criminalize, glamorize), it's worth noting that the vast majority of words that endinize do not experience this reversal of meaning.