Source : Webster's Revised Unabridged Dictionary (1913)

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 It has been superseded.

Source : WordNet®

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