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!"