[Thomas T]

I imagine the "pissing on arrows" thing is a "creative reinterpretation" of the old trick of dunking your lance/arrowheads in the cessppool, or a rotting carcass. The aim being that if your enemy survives his injuries, he's going to get some nasty infections and not be returning to the front any time soon. Pissing on arrows would seem to be a natural extension of the idea if you didn't know that that wouldn't work.

(The cesspool trick itself may well be apocryphal - I can't scare up any reliable refrences - but I suppose they could have heard of it in the same way that I did.)