I'm Y and I'm Proud
A: We should do X.
B: If you do X you are a Y.
A: That's not true!
B: The dictionary definition of a Y is someone who does X.
Of course, many of you spot the problem with this argument right away, but, even those who can see this in the abstract fall prey to the compelling nature of such argument when they employ it themselves.
Broadly speaking words have two sets of meaning. Denotations, which are direct and literal, and, connotations which are various degrees of hidden but just as powerful if not more so. Some words in the dictionary have the nature of their connotation(s) spelled out with labels like vulgar.
So let's deconstruct our argument, above. The advocate for X is labeled Y on account of her statement. X could be any number of things—"lower taxes", "stay home with our babies", "make union organizing easier", "legalize marijuana"—you get the idea. Her opponent responds with the label, Y and our advocate denies it.
Why? What is behind each person's behavior.
The person arguing against X would like to dispose of the idea as easily as possible. It is a challenging idea and probably complex. The advocate may well have a reasonable argument for it but the denier is ideologically opposed and doesn't want to be challenged.
The advocate objects to the label, Y. Why should she? Simply this, Y is selected not to describe but to discredit. Y admits of no nuance in the position of the advocate. Once labeled successfully as Y, our advocate can be dismissed out of hand.
Each party is paying much more attention to the connotations of Y than what it denotes. If someone has the terrible misfortune to be forced to kill in self-defense, Y can be "killer". While killer clearly denotes one who kills it also has negative connotations. If someone uses it in this context they intend to bring those connotations. If the labeled person objects they are not objecting to what it denotes but to the intended connotative content.
Arguments cannot actually be resolved with a dictionary (save those that are about attributes and content of the dictionary). However the dictionary is an excellent shield for those who would not be challenged by what is actually going on. "Words mean things" is a vapid retort to those who would prefer to deal with the intent of the speaker than with some orthodoxy that demands we own the connotations of words just because that happen to also denote something that matches. Worse, these same people will complain about neologisms. In other words they seek to control the content of our world by limiting the vocabulary available to describe it to an arbitrary authority.
Of course, all of my complaining just "semantics"...
1. The branch of linguistics and logic concerned with meaning. There are a number of branches and subbranches of semantics, including formal semantics, which studies the logical aspects of meaning, such as sense, reference, implication, and logical form, lexical semantics, which studies word meanings and word relations, and conceptual semantics, which studies the cognitive structure of meaning.
2. The meaning of a word, phrase, sentence, or text