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1.
A term you refer to when a situation requested by someone has been already done on The Simpsons.

Derives from South Park.
"How about we split the UrbanDictionary into two websites to adapt to the new area code?"
"Simpsons did it!"
by Mister Ignorant May 11, 2004
 
2.
A magic wand waved by a fan of a particular show to make accusations that that show ripped off another show disappear.

The line originally appeared on a South Park episode in which a foolish character kept shooting down another's ideas, dismissing them as ones the Simpsons originated, a point he made by annoyingly shouting "Simpsons did it!" The character was straightened out by a wiser character who explained that the Simpsons itself borrowed from other shows--the implicit premise being that everyone steals so no one should complain about it. The unfortunate result is that many people who look to South Park to know what their opinions are, now think that it's foolish to call people on plagiarism.

Now the line can be used more generally to ridicule anyone who points out that an episode of one show is similar to an episode of an older show.
Simpsons fan: Hey, that South Park episode is awfully similar to an episode of The Simpsons that aired last year.

South Park fan: Yeah, whatever. 'Simpsons did it!!! Simpsons did it!!!' That's what you sound like. Ha ha!
by Fusion Hungry Bagel Planet September 21, 2009
 
3.
A South Park episode which made fun of the simpsons doing everything.
Simpsons Did it, Simpsons did it, Simpsons Did it.
by John Smith the third April 17, 2006