1 [calthrop]. Orig. a snare; then, a spiked iron ball thrown on the ground to cripple horses in battle. Its principal element is still to be found in use today as the spiked metal strip used by police forces to puncture the tyres of miscreant car drivers. It was named after a plant called the 'water caltrop' (Trapanatans), also known as 'water chestnut', which entangled legs in its underwater leaves and tendrils. Caltrops were also used by poachers to disable deer.
Cf. Laqueus
2) Her. A *charge showing a ball with four spikes. Cambria. Wales. [< Welsh Cymru = Wales]
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.