interesting synonym for 'vicious' in one thesaurus. Interesting to see the equine connection.

I think it's the nails in the shoes, wordwind.

If you walk all over someone with nails in your shoes, that's vicious, real "vicious".

The Viet Cong used "naily boards" against their enemies. They would rather maim than kill because it put two additional soldiers out of action attending to the wounded.

Vietnam era U.S. Army Footwear

Standard black leather combat boots proved hot and susceptible to rot. The military introduced jungle boots, which included cooler nylon-mesh uppers, and drain holes that allowed water to escape. The boots also featured reinforced soles to protect against the sharpened bamboo stakes, or "punji spikes," used as booby traps by enemy soldiers.

http://www.pbs.org/wgbh/amex/vietnam/trenches/weapons.html