"Slipper" is a misnomer, as the usual item of footwear used was the plimsoll, or gymshoe or tennis shoe, with a fabric upper and a heavy rubber or synthetic sole.This could deliver a very painful stroke, depending on the force with which it was used, and the size and flexibility of the particular slipper; a size 11, well-worn, flexible slipper would deliver more sting than one that was new and stiff.