What does calthrop mean?
A calthrop is a small, typically four-pointed iron device designed to be strewn across ground, roads, or fords as an anti-cavalry and anti-personnel obstacle. Because of its shape, whichever way it lands one spike always projects upward, laming horses and wounding feet — a principle so effective it has persisted from ancient warfare through to modern tyre-puncturing versions. The term also extends botanically to prickly plants whose low-growing, spiny forms evoke the metal original, notably puncturevine (Tribulus terrestris) and the aquatic water calthrop (Trapa natans). The word derives ultimately from Latin calcitrapa, literally 'heel-trap', via Old English calcatrippe. Today "calthrop" survives mostly as a historical or regional spelling of "caltrop", which has become the standard form, and the word carries strong connotations of ambush, defence, and deceptively simple but painful ingenuity.
A spiked metal obstacle, usually with four points radiating from a centre, thrown on the ground so that one spike always points upward; used historically in warfare and later against vehicles.
"Garrisons poured boiling water on attackers while calthrops littered the approaches to the gate."
Often written as the standard variant "caltrop".
A prickly plant resembling the spiked device, especially puncturevine (Tribulus terrestris) or water calthrop (Trapa natans).
"Barefoot children learned to avoid the patches of calthrop growing along the riverbank."
Regular plural; used when referring to multiple devices or plants.
"The engineers swept the bridge carefully before crossing, wary of hidden calthrops."
The calthrop hasn't changed since Roman times: four spikes arranged so that no matter how it lands on the ground, one always points straight up at your foot.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of calthrop
The word descends from Old English calcatrippe, borrowed from Latin calcitrapa, a compound of calx ('heel') and trappa ('trap') — literally a 'heel-trap'. The same Latin source underlies related Romance forms such as French chausse-trape. The plant senses arose by analogy, since the spiny creeping stems and fruits of species like Tribulus terrestris resemble the iron device. The spelling "calthrop" reflects a later English reshaping of the older form, alongside "caltrop".
How calthrop is actually used
The spelling "calthrop" is a less common variant of "caltrop", which is now standard in both British and American English. The word appears chiefly in military history writing and in botanical contexts for the spiny plants.
Easily confused with calthrop
"Caltrop" and "calthrop" are essentially spelling variants of the same word; "caltrop" is the far more common modern spelling.