Source : Webster's Revised Unabridged Dictionary (1913)

Thundering \Thun"der*ing\, a.
   1. Emitting thunder.

            Roll the thundering chariot o'er the ground. --J.
                                                  Trumbull.

   2. Very great; -- often adverbially. [Slang] --
      {Thun"der*ing*ly}, adv.