To hear the common usage of this word (in the phrase '
shut your pie hole') you'd have to imagine it must have come to mean '
mouth' simply because that is the 'hole' (e.g. bodily orifice) which one would use to consume
pie. But in actuality, the word 'pie-hole' derives from an
Eighteenth Century Scottish term for the holes through which
laces were drawn in
boots -- which openings were initially simply called
eyelets or 'eye-holes,' owing to their similarity to the eye of a needle. And which phrase probably got changed over from eye-hole to pie-hole simply owing to a rhythmic similarity of sound. The 'p' it is noted, might have come from
pig or
pyg, which resembled the Danish word for a
prick (meaning a small hole, bootlace holes being made by using a leather punch to 'prick' the boot). And so by the mid
Nineteenth Century the word 'pie hole,' because it represented this little tiny hole for shoe-lacing, came to be used euphemistically among the Scottish to reference any really small hole, especially anything which would be a tight fit for a person to squeeze through for example (as in, the boy could barely squeeze through that pie-hole in the fence). Now, when and how it came to refer to the mouth (and especially to the mouth which somebody was commanding to be shut), well that remains a
mystery. But you can bet it began with the Scottish.
One additional note on usage: it appears that when instructing somebody to shut their pie hole, it is most amusing to pronounce/spell the possessive pronoun as "yer" -- as in, "shut yer pie hole, ye scalliwag!!"
Reference:
An etymological dictionary of the Scottish language: illustrating the words in their different significations. Volume 2. By John Jamieson, Edinburgh, 1808.