The Collaborative International Dictionary of English v.0.48:

Petard \Pe*tard"\, n. [F. p['e]tard, fr. p['e]ter to break wind,
   to crack, to explode, L. pedere, peditum.] (Mil.)
   A case containing powder to be exploded, esp. a conical or
   cylindrical case of metal filled with powder and attached to
   a plank, to be exploded against and break down gates,
   barricades, drawbridges, etc. It has been superseded.
   [1913 Webster] Petardeer

WordNet (r) 3.0 (2006):

petard
    n 1: an explosive device used to break down a gate or wall