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1.
The act of defecating in the back tank of a toilet. The result is that whenever there is a flush, the bowl fills with tainted water.
Before I moved out, I top tanked the toilet in the master bedroom.
by Robert Smith August 06, 2004
 
2.
The act of shitting in the cistern of an enemy's toilet to ruin their bathroom for a thousand flushes.
Shortly before they sealed off Hitler's bunker beneath Berlin, Larry Silverstein toptanked every toilet available and made good on his escape.
by Kenny Evil June 30, 2010
 
3.
Taking a dump in the systern of a toilet. Holding youself up by you hands on the wall and dropping in a bombay bad boy. The only way that can be fixed is if a glass-monkey mops the whole stinky area out.

Particularly skilled people can piss at the same time and even sometimes get a few drops in the bowl.
Norm gave Gilhooley a top tank for his birthday. He thought and animal (a trained dog) had done it.
He was wrong.
by Matt Regsmith April 30, 2003