A metal device with four spikes arranged so one always points upward, used historically as a trap to puncture tires or injure people; also, a plant with prickly seed pods.
From Old English 'caltroppe,' possibly from Latin 'calcitrappa,' combining 'calx' (heel) and 'trap,' describing its punishing effect on feet.
Calthrops were medieval psychological warfare—scattering them on roads created terror because horses and soldiers dreaded them, even if only a few were actually there!
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