HEEL TRAP

The word caltrop describes a type of spiked metal object that, when planted on the ground, is useful in defending against enemy cavalry or puncturing automobile tires. When it was first used in English in 1300, it was spelled calketrap; subsequent forms included calcatrippe, caltroppe, calltrop, calthrap, galtroppe, and many others. Even today, there's no single standard spelling, with caltrap, galtrop, galthrap, galtrap, and calthrop all accepted by different authors. It all comes from the Medieval Latin word calcatrippa, meaning "thistle". That's composed of calx, or "heel" (also the etymon of calcium, calculation, and causeway), and trappa, "trap". Calx, which was likely influenced by the verb calcare ("to tread"), comes from Ancient Greek khalix, meaning "pebble", and trappa traces to the Proto-Indo-European root dremb, meaning "to run".