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]