Commentary: I first heard this expression from my boss describing an all too common phenomenon in Shanghai.  While walking on the streets here, unexpectedly and sometimes only for a moment, it will smell as though your head has been violently forced into the holding chamber of a port-o-potty.  There is a mysterious element to an ass pocket, as the source of the smell is never seen.  It should be noted that unlike America, when one walks through a shit-smelling zone in China, it is almost certain the excrement is of the human variety.  Many Chinese apartment buildings do not have sufficient plumbing forcing residents to dispose of all fecal matter themselves.  Moreover, China has a history of using human excrement for fertilizer, sometimes even for house plants.  Waste not, want not.