“Shut your piehole.”
“Shut your cakehole.”
OK, which is ruder?
Obviously, both are rude. Even “shut your bouche” would be rude, though also confused. But there’s something particulary nasty about piehole and cakehole. If we see them in a context other than shut your, it’s likely to involve a word such as cram or stuff: “He gobbled down as much as he could cram into his cakehole.” “He stuffed it quickly into his piehole.” Want further evidence that these are low, rude words? Compare this: “The queen took the most delicate forkful of camembert soufflé and placed it delicately in her cakehole.” Did you laugh? Proof: the contrast is absurd.
It’s fairly plain to see that hole is a rude word when applied to a mouth. A hole, after all, is a simple, round, inarticulate thing; other holes we have on the body are the earholes and the nostrils (which have a non-hole name) and one other hole, an especially vulgar one. The resonance is clear. Hole is, after all, a plain old Anglo-Saxon word, of the stock that for a few hundred years was associated with the ruder folk while the court preferred French and the scholars used Latin.
So is cake. It has cognates throughout Western European languages, especially Germanic and Balto-Slavic ones. Of course, cake is delicious – everyone loves cake, and some people gobble it greedily, cramming it in their cakeholes. But cake is also a verb that is not always pleasant. What sort of thing gets caked on? Exactly.
Pie, on the other hand, while having been in English since medieval times (think of Simon the pie man, from the nursery rhyme), is not an Anglo-Saxon word in origin. In fact, it appears to come from Latin originally, by way of a bird. A bird? Not four-and-twenty blackbirds, no; a bird that is famous for collecting all sorts of odd things: the magpie, formerly called just the pie, from Latin pica by way of some later Romance languages. It seems that a pie was conceived of as a sort of omnium gatherum dish (mincemeat comes to mind).
Pie has long been a popular dessert, of course, and in many ways a demotic one: there’s the old story of a royal visiting a small town in western Canada and being told, after the main course, “Keep your fork, Duke, there’s pie.” Not “Keep your fork, Duke, there’s cake,” though that would give a more consistent sound. No, cake is a more elevated thing: remember, Marie Antoinette was supposed to have said, in an oblivious insult, that peasants without bread could eat cake (actually brioche in the original). (This line had earlier been attributed to others, and most likely no one ever said it as such.) “Let them eat cake”? How rude. But “Let them eat pie”? Mmm, delicious. It’s true that royals have eaten lamprey pie (and many other kinds), but the associations are established: cake is a loftier dessert, but the word also has more unpleasant overtones, which lend further to cakehole. (There is also possible effect from K-hole, a bad trip caused by a particular party drug colloquially referred to as special K, but that’s in more limited circulation.) Cake is also a harder word, with those /k/ and /k/ stops kicking at front and back. Pie starts with a pop and then fades out.
In any event, cakehole is somewhat older – it dates from at least the early 1900s, while piehole only starts showing up in the 1960s and then more in the 1980s and on – and cakehole is by a long measure the more used of the two. I would say it’s also the ruder. If you disagree, just cram something in your piehole and shut your cakehole.