The Collaborative International Dictionary of English v.0.48:

Trocar \Tro"car\, n. [F. trocart (or trois-quarts, i. e., three
   quarters); trois three (L. tres) + carre the side of a sword
   blade; -- so called from its triangular point.] (Surg.)
   A stylet, usually with a triangular point, used for exploring
   tissues or for inserting drainage tubes, as in dropsy.
   [Written also trochar.]
   [1913 Webster]