A Grammatical Dictionary of Botanical Latin

Tribulus,-i (s.m.II), abl. sg. tribulo: a genus Caltrops (Tribulus terrestris L.), used by Vergil; a kind of water-plant bearing a prickly nut of a triangular form, water-chestnut, water-caltrops (Trapa natans L.), used by Pliny (Lewis & Short) [> L. tribulus,-i (s.m.II)] “Latin name of the caltrop, the form of which is suggested by the prickly fruit” (Fernald 1950); see Bur; see Morning-star; see caltrop.

Tribulus,-i (s.m.II) L., T. terrestris L., common name ‘Caltrop,’ the “mature carpels crested and armed with 2-4 spreading prickles” (Fernald 1950).

Note: Trapa,-ae (s.f.I) L., the Water-Chestnut is a “Name abridged from calcitrapa, a caltrop, in allusion to the spreading points of the fruit” (Fernald 1950); > L. calcitrappa, “called a caltrop or crow’s foot - a weapon of defensive war with four sharp iron points which, thrown on the ground, has one point always pointing upward particularly to pierce the hooves of cavalry horses” (Stearn 1996).

“From Old English calcatrippe (“plant that trips”), from Medieval Latin calcatrippa (“thistle”), from Latin calx or calcare + trappa (Wiktionary, Oct. 2017).

Centaurea calcitrapa, Star-thistle, Caltrops, “old name, a caltrop” (Fernald 1950). from the spines on the coriaceous phyllaries.