Cutie is a slang term used to refer to someone who is cute.
Where did the word, pie, in the expression "Cutie Pie" come from? And what is the history behind this expression?
I can't seem to find any references to it.
The term seems to be in use before Nicholas Fisk wrote his story so it may be that he popularized the term rather than coining it. The earliest reference I could dig up was a pony named "Cutie Pie" who won a modest sum in a show ponies convention in 1919, so it's possible that the term was in use as a diminutive nickname, perhaps to mean something like cute + sweet (as a pie).
The Oxford English Dictionary points out a reference from a year later of a cat called "Cutie Pie" mentioned in the Washington Post. They do not attempt to provide an etymology however, other than to say that "cutie" is a derivative of "cute" and then to point to the phrase "sweetie-pie" as probably related. Like "Cutie Pie" the earliest instances I could find was as the name for a pet (dog in this case) in 1922.
I don't know what the pie means, but the phrase dates at least to November 28, 1916, as found in The Washington Herald:
(Characters-Harry th' barkeep over th' phone with someone in Washington.)
Someone-Hello! Is 'at you, Harry? Well, say! Listen! Listen, kid! Ain't It no way you can get off Thanksgivin' an' take me over t' see th' football at Georgetown?
Harry-Not a chance, Cutie-Pie! I'm still in th' hands o' my Oculist!
Cutie Pie is story by Nicholas Fisk. It is a story included in several high schools' English syllabus. Google, "Cutie Pie by Nicholas Fisk" and you'll get the answer.