Verb: "to top deck".
To "top deck" is to defocate into the cistern of a toilet. The cistern is the water-filled tank often found above the toilet seat, near the flushing handle. Once the cistern has been defiled with faeces, the toilet will begin to flush "shitty" water of a brown, slimy appearance. This not only leaves the victim with a very nasty clean-up job, but more importantly it represents an extreme hatred or humiliation for them.
The art of topdecking is something commonly joked about but very infrequently practised. It is a rare and extreme example of domestic terrorism, often done as a form of revenge or simply to cause gross inconvenience to the victim.
"Hey, give that back or I'll top deck in your toilet!"
"Man, I'd love to top deck Oxford University..."