The floodgate turd is a piece of poop which holds back a torrential flow of diarrhea that has built up in a persons colon. Visually similar to rushing water flowing through a dams floodgates.
"I was pounding away my girlfriend with her bent over her bed, right? Well she's a squirter, so when I pulled out it looked and sounded like someone fucking threw a bucket of water onto the floor between her legs. I've been calling her floodgate ever since."