caltrop

noun

Etymology: From Middle English calketrappe, from Old English calcatrippe, from Medieval Latin calcatrippa (“thistle”), from Latin calx or calcare (“to tread”) + trappa (“trap”).

1. A metal object, usually small, with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles (also used as a heraldic charge).

   “[…] her father, the emperor Alexius, who reigned AD 1081-1118, ordered caltrops to be cast in front of his archers […]”

   “By Sung times, several different types of caltrops had been developed. As in earlier times, both caltrops could be made from both wood and iron...”

2. The starthistle, Centaurea calcitrapa, a plant with sharp thorns.
3. Any of a number of flowering plants in the family Zygophyllaceae, including several members of the genus Kallstroemia and the species Tribulus terrestris, native to warm temperate and tropical regions.