caltrop

1. (weaponry) 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).Examples: "[…] 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..."; "Caltrops, tetrahedrons, and similar devices are designed to puncture vehicle tires or limit foot traffic. The standard design has four points."

2. (colloquial) The starthistle, Centaurea calcitrapa, a plant with sharp thorns.Synonyms: knapweedcolloquial