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The act of sitting on a toilet backwards, so your knees touch the wall behind the toilet, and pushing excrement through the anus, so that it spills down the front of the bowl and stains it really badly.
John: For fuck's sake, Mark, I can't go into that toilet.
Mark: Why, John?
John: Some prick did a Reverse Canker in there...
by jack french November 08, 2006
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