The sexual act of deficateing your partners mouth and then receiving oral sex giving the male appendage the look of the classic chocolate covered banana treat the monkey tail
Yo I pooped in this chick's mouth and she gave me a monkey tail
When you wait too long to poop and a turd starts to extend past your butthole involuntarily. Similar to a turtle head or a prairie dog, but possibly describing a more severe scenario.
Wait; I gotta call you back from the toilet. I’ve got a monkey tailsituation going on.