Wainscot derives from the 1300’s (The Middle Ages) . It was originally not an over application, but an instead of application. Wood board paneling would be cheaper than plastering, which was labor intensive, used 3/4 of the amount of wood anyway and horsehair to hold the plaster together. Now we use relatively cheap gypsum board and dress it up with thin veneers instead of real boards to simulate what people used to do out of low tech necessity.
Here’s a definition from http://www.m-w.com.
Main Entry: 1wain·scot
Pronunciation: \ˈwān-skət, -ˌskōt, -ˌskät\
Etymology: Middle English, from Middle Dutch wagenschot, probably from wagen wagon + schot shot, crossbar
Date: 14th century
1 British : a fine grade of oak imported for woodwork
2 a (1) : a usually paneled wooden lining of an interior wall (2) : a lining of an interior wall irrespective of material b : the lower three or four feet (about one meter) of an interior wall when finished differently from the remainder of the wall