A females anal cavity that has been penetrated by the male reproductive member enough to stretch out its "elastic" surround causing it to become saggy and loose.
Damn that bitches ass been fucked so much shes got a sinkhole!
Person A gets on the top of a bunk bed and proceeds to strategically aim his solid fecal matter into the open orifice known as the mouth and or nose of person B, who is sprawled eagerly on the floor below.
Last night I dropped a sinkhole right in Susan's mouth. It was hot!