That which is said to he “in apple pie order” has a perfection of orderliness, but no one has been able to figure out the reason for this with certainty.
The expression is of British origin, and has been used since early in the nineteenth century at least. Some have tried to figure that it may have been derived from “cap-a-pie,” which, in English usage, means “from head to foot,” but there is no known instance of such an expression as “cap-a-pie order,” and the expression itself seems meaningless.
A very recent philologist, Bruce Chapman, asserts confidently: “The phrase comes from the French ‘nappes pliees’, meaning ‘folded linen,'” but he does not cite any instance of the use of this French phrase in English literature, and, unable to find any instance myself, we cannot support his derivation, plausible though it may appear.
If only some cookbook of, say, 1800 could be found, possibly we might learn that some unusually finicky cook had prescribed a most precise manner or arrangement in which an apple pie should properly be concocted, so precise as to earn the derision of all proficient cooks who thereupon made “apple-pie order” notorious.