"Beetchabacks" are small bugs that live in humans' anal cavities.
They crawl out of the anus and bite the ass in various places causing itchiness.
Then, when the subject goes to scratch the bites, they "beetchaback" to the anus.
This is where the creatures get their name.
"That fat guy over there has been scratching his ass for the last fifteen minutes. I bet he has a wicked case of Beetchabacks."