[cal.trop] also cal.throp n [ME caltrappe, alter. of calketrappe star thistle, fr. OE calcatrippe, fr. ML calcatrippa] (15c) 1 a pl but sing or pl in constr: star thistle 1 b: puncture vine; also: any of various related herbs (genera Tribulus and Kallstroemia) of the same family (Zygophyllaceae, the caltrop family)
2: a device with four metal points so arranged that when any three are on the ground the fourth projects upward as a hazard to the hoofs of horses or to pneumatic tires