trous-de-loup

from The Collaborative International Dictionary of English v.0.48

Trou-de-loup \Trou"-de-loup"\, n.; pl. {Trous-de-loup}. [F. trou
   hole + de of + loup wolf.] (Mil.)
   A pit in the form of an inverted cone or pyramid, constructed
   as an obstacle to the approach of an enemy, and having a
   pointed stake in the middle. The pits are called also
   {trapholes}.
   [1913 Webster]