Is there a term for a word that defeats its own purpose?
There's a makey-up word, heterological.
see http://www.segerman.org/autological.html
Paradox is actually the word you want, though specifically a logical paradox.
From the article:
- Self reference – An example is "This statement is false", a form of the Liar paradox. The statement is referring to itself. Another example of self reference is the question of whether the barber shaves himself in the Barber paradox. One more example would be "Is the answer to this question no?" In this case, if you replied no, you would be stating that the answer is not no. If you reply yes, you are stating that it is no, because you said yes. But because you answered yes the answer is not no. However you could reply "It isn't." indicating a negative response without saying the word "no".
- Contradiction – "This statement is false"—the statement cannot be false and true at the same time.
Vicious circularity or infinite regress – "This statement is false"—if the statement is true, then the statement is false, thereby making the statement true. Another example of vicious circularity is the following group of statements:
"The following sentence is true." "The previous sentence is false."
You really can't get much more accurate than the actual definition, paradoxical is probably the best word to use.
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.
maybe Contronyms or Antagonyms or Auto-antonyms?
Many words have several definitions, like bill. But in some cases those definitions are conflicting. With bill, for example, in one instance it refers to a debt and in another an asset. These are called auto-antonyms. (They are also called “contronyms,” “antagonyms,” “self-antonyms,” “self-contradicting words,” and “janus words.”)