While I cannot say that the meaning of "stookie" as plaster of Paris is wrong, it is certainly not one which I came across in 55 years in Scotland, including use of plaster of Paris as a schoolchild.
It is a country/farming term (of Germanic origin and probably related to "stack") from the days before mechanisation; the "-ie" ending is a diminutive used for smallness or familiarity, so the change from "stucco" to "stookie" would not be typical.
So, a stook is a stack of hay or grain and a stookie is a bundle of hand-gathered sheaves of corn (eg oats, barley, wheat) tied together and left standing up on their stalk ends in the field to air-dry and ripen, before threshing or building into a stack. This gives them the approximate shape of a person, hence the meanings given as an idiot or quiet, reticent person. Example of use - "Don't stand there like a stookie, boy, speak up!"
Cosmite 17:27, 22 January 2012 (UTC)