Caltrop

n. any of several species of plants with spiny heads or fruit; iron ball with four spikes arranged so that one is always pointing upward (used to hinder passage of cavalries or vehicles)

A caltrop (calthrop, jack rock, star nail or crow's foot) is a weapon made up of two (or more) sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron). Caltrops serve to slow down the advance of horses, war elephants, and human troops. It was said to be particularly effective against the soft feet of camels. In more modern times, caltrops are used against wheeled vehicles with pneumatic tires. In Japan such devices were known as makibishi.
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Noun
1. tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed
(synonym) devil's weed, Tribulus terestris
(hypernym) subshrub, suffrutex
(member-holonym) Tribulus, genus Tribulus
2. a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits
(synonym) water chestnut, water chestnut plant
(hypernym) aquatic plant, water plant, hydrophyte, hydrophytic plant
(hyponym) water caltrop, Jesuits' nut, Trapa natans
(member-holonym) Trapa, genus Trapa
3. Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America
(synonym) star-thistle, Centauria calcitrapa
(hypernym) centaury

(n.)
Alt. of Caltrap
  

Sharp spikes, resembling ‘jacks’ used to maim a horse. Robert the Bruce used them at Bannockburn, and there are references to spurs being used for this same purpose.