I don't rightly remember when or where I first heard the term pie hole used. Probably in a movie or T.V. show, or quite possibly in a book. The latter is least likely as I just don't really that many books any longer (A.A.D.D. you know). It's just an amusing, and somewhat derogatory, term for one's mouth...the body opening through which an animal takes in food (such as pies), the cavity lying at the upper end of the alimentary canal, commonly regarded as the source for sounds and speech, etc.
The term pie hole might be appropriate in the following examples of colloquial expression...
"Hey, dipshit. Shut your f&%*ing pie hole and get back to work!"
"If that politician used his pie hole less and his ears more, he might get elected!"
"How'd you like me to shove my fist down that pie hole of yours?"
The term pie hole might be inappropriate in the following examples of colloquial expression...
"I still can't hear what you are saying. Open your pie hole and speak up please!"
"Oh my...the baby has put something dangerous in his pie hole!"
"Honey. You have the prettiest pie hole I've ever seen on a woman."
Pie hole is simply fun to say as well. Like the number 13. "I've told him 13 f*%#ing times to clean up his pie hole." I digress.
That's enough out of my pie hole for the time being. Tomorrow...maybe a closer examination into slang synonyms for buttocks and anus. Hmmm, let's see...caboose, rear end, poop shoot, ying yang, fart box...what fun we'll have with that one!