Hell for Leather

When I was learning to tie knots the term bend was used as the name for a part of a knot or a whole knot. For example, a sheet bend. I always took bend to mean knot. If one bends a rope to a spar that just means to tie it on. So bent would be like tied. Also as a horseman I know that saddle straps are called leathers. This leads me to speculate that the phrase might mean something like, hell being used to fasten ones self to the horse. I suppose the term leathers likely applies to other situations like soldiering.