caltrop
A metal object, usually small, with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles (also used as a heraldic charge).
[…] her father, the emperor Alexius, who reigned AD 1081-1118, ordered caltrops to be cast in front of his archers […]
Where it comes from
From Middle English calketrappe, from Old English calcatrippe, from Medieval Latin calcatrippa (“thistle”), from Latin calx or calcare (“to tread”) + trappa (“trap”).
Adapted from the Wiktionary entry caltrop (CC BY-SA 4.0).