Source : Webster's Revised Unabridged Dictionary (1913)

Topple \Top"ple\, v. i. [imp. & p. p. {Toppled}; p. pr. & vb. n.
   {Toppling}.] [From {Top} summit.]
   To fall forward; to pitch or tumble down.

         Though castles topple on their warders' heads. --Shak.

Topple \Top"ple\, v. t.
   To throw down; to overturn.

         He topple crags from the precipice.      --Longfellow.

Source : WordNet®

topple
     v 1: fall down, as if collapsing; "The tower of the World Trade
          Center tumbled after the plane hit it" [syn: {tumble}]
     2: cause to topple or tumble by pushing [syn: {tumble}, {tip}]