caltrop

(noun) tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed. Synonyms: devil's weed, Tribulus terestris.

(noun) a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits. Synonyms: water chestnut, water chestnut plant.

(noun) Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America. Synonyms: star-thistle, Centauria calcitrapa.