dragon's teeth

noun[usually plural]3

1. •
   Square-pyramidal fortifications of reinforced concrete to impede the movement of tanks and mechanised infantry.

   • Experiments showed that twenty-five to fifty pounds of explosives often was needed to break a single dragon's tooth, and large pillboxes required half a ton. — 2013
2. •
   (UK, Australia) A series of triangular road surface markings painted on each side of a traffic lane, used for traffic calming or to alert drivers to a speed limit.
3. •
   Lotus maritimus, a plant in the legume family.