I'm sure the urban dictionary link answered your question, but if you didn't go there for whatever reason, it's when you push a turd down your tub drain with your foot.
(And for the record, I didn't actually poop in my shower)
That's actually worse because it would be more likely to clog your drains. Your dude goo would be thicker than urine which is basically just water. If I remember correctly heat such as that from a shower will actually cause it to coagulate more.