from
The Collaborative International Dictionary of English v.0.48
Caltrop \Cal"trop\, Caltrap \Cal"trap\, n. [OE. calketrappe,
calletrappe, caltor (in both senses), fr. AS. collr[ae]ppe,
calcetreppe, sort of thistle; cf. F. chaussetrape star
thistle, trap, It. calcatreppo, calcatreppolo, star thistle.
Perh. from L. calx heel + the same word as E. trap. See 1st
{Trap}.]
1. (Bot.) A genus of herbaceous plants ({Tribulus}) of the
order {Zygophylle[ae]}, having a hard several-celled
fruit, armed with stout spines, and resembling the
military instrument of the same name. The species grow in
warm countries, and are often very annoying to cattle.
[1913 Webster]
2. (Mil.) An instrument with four iron points, so disposed
that, any three of them being on the ground, the other
projects upward. They are scattered on the ground where an
enemy's cavalry are to pass, to impede their progress by
endangering the horses' feet.
[1913 Webster]