A fictitious creek, that has shit flowing just like water would normally flow. As a figure of speech, one might say "I am drowning!", but if it worse than that, one may say "I am drowning in shitcreek".
If one creek did exist, one would probably have more of a chance of living after falling in shitcreek, than a normal creek, due to the viscosity of feces being higher. Most people usually do not think about this, because it’s not something you think about.