Is there a term for a word that defeats its own purpose?

There are a few words that can possess an oxymoronic or paradoxical meaning that tend to defeat the entire purpose of the word. The most obvious example:

This sentence is indescribable.

Specifically, words such as "indescribable." The paradox is that the description of something as "indescribable" means it now the opposite of "indescribable" but I am not interested in the paradox itself. I want to know what to call the word "indescribable" in the sense that word defeats its own purpose. By using the word it becomes inaccurate. Other examples seem hard to find, but this is close:

(spoken) How quiet!

In this case, "quiet" also makes itself suddenly inaccurate. The paradoxical nature is again obvious but what I am trying to find would be more akin to "self-referential meaninglessness" or "a word that makes itself inaccurate."

Answer

I found it:

contradict

• 
  

  deny the truth of (a statement), esp. by asserting the opposite

  
  


• 
  

  assert the opposite of a statement made by (someone)

  
  

self-contradiction

• the act, state, or fact of contradicting oneself

So a word that defeats its own purpose or definition would be a self-contradicting word. The act of using such a word changes the situation to contradict its own definition.