caltrop

US: (kal′trəp)

WordReference Random House Unabridged Dictionary of American English © 2026

cal•trop  (kal′trəp), n. 

1. Plant Biology any of several plants having spiny heads or fruit, as those of the genera Tribulus and Kallstroemia, or the star thistle, Centaurea calcitrapa.
2. Military an iron ball with four projecting spikes so disposed that when the ball is on the ground one of them always points upward: used to obstruct the passage of cavalry, armored vehicles, etc.

Also, cal•throp  (kal′thrəp), cal′trap. 

• Latin calci-, stem of calx spur, heel) + træppe trap¹
• Middle English calketrappe, Old English calcatrippe, colte-træppe, equivalent. to calce- (bef. 1000

Collins Concise English Dictionary © HarperCollins Publishers::

caltrop, caltrap / ˈkæltrəp/calthrop / ˈkælθrəp/ noun

1. water caltrop ⇒
   another name for water chestnut1

Etymology: Old English calcatrippe (the plant), from Medieval Latin calcatrippa, probably from Latin calx heel + trippa trap¹