Sentence:  When I went to Harrow, thirty years ago, I found a winter evening game in force there, called 'high <WSD>cockalorum</WSD>,' ... The players used to divide into two opposing bands of from twelve to fourteen each – in fact, the more the merrier. One side 'went down,' so as to constitute a long 'hogsback' – the last boy having a couple of pillows between himself and the wall, and each boy clasping his front rank man, and carefully tucking his own 'cocoa-nut' ["i.e.", head] under his right arm, so as to prevent fracture of the vertebrae. When the hogsback was thus formed, the other side came on, leapfrogging on to the backs of those who were down, the best and the steadiest jumpers being sent first. ... 
Meaning of the word marked with <WSD> tags:  A game similar to leapfrog.