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1.
It is pretty much the greatest response to anything. Can be used for the following reasons:
1. The other party asks a question so retarded and obvious it doesn't merit a serious response.
2. Someone eavesdropping on a conversation asks a question about it.
3. You don't know the answer but know what your saying is true.
4. Some annoying brat is asking questions just to try and stump you.
5. just improv it into any situation for comedic value or to silence someone you don't like.
Example 1:

Guy 1: Why do people think Hitler was so bad?
Guy 2: Because fuck you thats why.
Example 2:

Guy to his buddy: So ya me and my girl were at the movies and we were going at it hardcore and...
Eavesdropping douche: Why wern't you paying attention to the movie?
Guy: Because fuck you thats why.
Example 3:

Guy: You see for every action there is an equal and opposite reaction.
Douche: why?
Guy: Because fuck you thats why.

Example 4:
Guy: Do you have any questions son?
Brat: Whys the sky blue?
Guy: Because the suns rays break up in the atmosphere and you only see the blue?
Brat: why?
Guy: Well because your eyes can only see certain colors.
Brat:why?
Guys: Well because your eyes laid out in a specific way that only allows that.
Brat: why?
Guy: Because fuck you thats why.

Example 5:
Girl: Why dont we just stay home and cuddle?
Guy: Because fuck you thats why.
by MC J.Bizzle March 28, 2012
 
2.
another way of saying "because I felt like it"
Usually used when describing something that could've been done more practically.
I disconnected the natural gas input from my stove and lit it on fire because fuck you.

I needed to walk five blocks to the library, so I stood on the back of a semi-truck going the same direction because fuck you.
by Herpderpdederp May 17, 2012