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1.
A house guest who defecates in one's host cistern, rather than, more conventionally, in the toilet, for comic effect.
Tony Blair and his wife Cherie were invited round by the Browns to congratulate Gordon on his appointment as Prime Minister. Cherie was furious because she never liked Gordon and wanted her Tony to stay on another 10 years. She said to herself : “I know, I'll fix 'em with a good smelly top-decking after the main course”.
by humf May 15, 2008
 
2.
The deplorable act of taking a shit in the cistern as opposed to the toilet bowl, hence taking a dump in the 'top deck'.

Usually performed as a form of revenge.

Also known as slam dumping
This party sucks. I'm gonna topdeck the shitter and then let's get the fuck out of here.
by mark August 31, 2004