Take the word "theft" for instance; what does it mean? What does it imply? How is it used? How is it properly used? People like Gene Callahan contend that when libertarians say that taxation is "theft" it's justified only via some form of circular argument in which we have to already assume the premise that it is theft. In other words, he believe that if what taxation takes is rightfully owned by others, then it's not theft - and certainly not wrong in the criminal sense. He also points to what he sees as a somewhat lacking consideration of how we use the terms "force" and "violence" when we allow for those things as a matter of justice.
This brings up an interesting point - one that I'm agreeing with more and more over time - but one that certainly doesn't overturn the libertarian argument. When libertarians use "theft," "force," and "violence" they mean something very specific - the wrongful violation of one's property rights. So more specifically we should talk in terms of property rights if we'd like to make the conversation(s) less confusing or contestable.
That being said, I think it's perfectly OK for libertarians to use such terms (particularly "theft" and "aggression" among others) because I believe that, properly understood, these terms approximate the meanings libertarians give them even in much of their everyday usage. In other words, if you break the terms down into their more literal implications, it turns out the libertarian usage is the common use - with a few exceptions (taxation being one of them). The implication of theft is a violation of property rights. A violation of property rights presupposes property rights. To talk of theft, in any capacity, is to refer to a system of property rights. Therefore, in order to justifiably, and forcibly, take something from someone you must establish that you already own it somehow.
Now, some like Callahan might contend that you could have such a view about taxation - that government really does own the quite literal property rights to the product of your labor and trade. But, without getting into the philosophical framework of property itself, I'd contend that most people don't see it that way. I think that people wholeheartedly believe in positive obligations of all sorts, but I think very few of them view the first X hours of your labor as actual property of government per se - rather they feel you have a positive obligation to sacrifice some of your property to the greater good.
Again, I think the conversation might be even further confused in the conflated usage of terms like "rights" to begin with, but I think it's interesting that so many of our deepest quarrels seem to be over semantics in some real way. I think that's my queue to start diving into philosophers that focus on linguistics. I'm thinking Wittgenstein isn't a bad start...