caltrop

1. a spiked metal device thrown on the ground to impede wheeled vehicles or (formerly) cavalry horses.

   militant demonstrators threw a box of caltrops across the highway

2. a creeping plant with woody carpels that typically have hard spines and resemble military caltrops.
3. another term for water chestnut (sense 3).

Old English calcatrippe, denoting any plant which tended to catch the feet, from medieval Latin calcatrippa, from calx ‘heel’ or calcare ‘to tread’ + a word related to trap¹. caltrop (sense 1) was probably adopted from French.