reachthestars: it is pronounced 'shaps' b/c it is an abbreviation of a Spanish word 'chapparerras' and the 'ch' in Spanish is pronounced 'sh'. The word means something like 'leg armor'.
We have always called what you call English chaps 'schooling chaps'. I have a pair (that still fit!) that I got when I was about 15 but I much prefer my Hobby Horse ultra suede fringed ones (and yes I also ride english!)