What is "apple-pie bed"

Apple pie \Ap"ple pie`\ A pie made of apples (usually sliced or stewed) with spice and sugar.

Apple-pie bed, a bed in which, as a joke, the sheets are so doubled (like the cover of an apple turnover) as to prevent any one from getting at his length between them.
--Halliwell
--Conybeare.

Apple-pie order, perfect order or arrangement. [Colloq.]
--Halliwell.

n. (context British English) A shortsheeted bed in which a sheet is folded back on itself halfway down as a practical joke so that the victim cannot get into it.