A response to a question of the practicality of something which is clearly overkill, it implies that the one who uses the phrase to answer the question of 'why' knows full well that it is overkill, and overkilling for overkill's sake was their intent.

(The examples probably describe it better)
Q: Why would you build an aircraft carrier that flies?
A: Because fuck you, that's why

Q: Why would you need a 6-core CPU?
A: Because fuck you, that's why

Q: Why would you make an automatic grenade launcher?
A: Because fuck you, that's why
by gnerd October 11, 2011
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The correct response to a smart arsed sarcastic question, usually from your partner.
Gemma :"You have apparently took a wrong turn and are now going in the wrong direction. Phils house is the other way, why did you do this?"
John : "Fuck you that's why!"
by KingPryle September 23, 2010
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This is what you say after you punch and/or kick a guy in the balls. This is a good revenge to use on douchebags. Here's what you do:

Knock on douchebag's door.

When douchebag answers, come at him like a spider monkey- a deranged, rabid spider monkey.

Punch/kick him right in the nuts.

As he writhes on the floor and cries, "Whhhhyyyy??!?!?!", You point at him and yell very scarily, "YOU KNOW WHY!!"

And he will know why.

Revenge accomplished!
How to correctly carry out a "you know why."

Dave (answers door): Oh, hi..
(Sara, crazy eyes and all, punches Dave in the nutsack)
Dave (falling to the floor): WWWWWHYY?
Sara: YOU KNOW WHY!
by ,fknkd April 25, 2010
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An exaggerated statement usually following a junk punch; including a gesture of an angry pointing of the finger and an evil eye.
*Knock on door*
*Douchebag answers*
DB: ..You!
*Junk puncher punches junk*
DB falls to his knees: WHY?!
JP: You know why.
by pinkdaisy4 February 10, 2009
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often said when someone does or says something stereotypically Maori, such as stealing or going to KFC.
you know why... its because your Maori.
Nathan: bro i got me a mean feed at kfc last night
Josh: you know why...
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When you are given reasons that justify you cheating on someone. Reasons could include: someone thinking they are smooth, short-term memory, or just stupidity.
"Yeah, you fucking wonder why i've been cheating on you." "Keep being a fucking idiot... you wonder why."
by Hives! May 5, 2005
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Douche: yo, why can't I get some of that azz?

Hot chick: because fuck you that's why.
by Supadupaflye69 August 12, 2014
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