A large, heavy turd. So named for the "t" that starts the word "turd" and the "explosion" it creates upon hitting the toilet water. T-bombs are often the result of massive consumption of constipation-causing foods, and they are often referenced in tandem with a city that has famously been bombed (Hiroshima, Baghdad, etc).
"I knew I shouldn't have drank that case of beer. I just dropped a five megaton t-bomb on my toilet and now it looks like Dresden!"