A fetus that has miraculously transferred to the anal intestinal, thus being able to give birth to the baby through the anus. This happens once in 32 trillions, though it hasn't happened or at least noted in the anthropological history.

In the 21st century the word evolved into a slang for having a huge amount of excrement in the rectum, and therefore having the need to dispose it in the nearest future.
"I had to shit so bad, i felt like i had an anus fetus!"

"Some history books say that Jesus himself was in fact an anus fetus!"
by The Crazy Madman November 4, 2011
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