Source : Webster's Revised Unabridged Dictionary (1913)

Thunderous \Thun"der*ous\, a. [Written also thundrous.]
   1. Producing thunder. [R.]

            How he before the thunderous throne doth lie.
                                                  --Milton.

   2. Making a noise like thunder; sounding loud and deep;
      sonorous. -- {Thun"der*ous*ly}, adv.

Source : WordNet®

thunderous
     adj 1: loud enough to cause (temporary) hearing loss [syn: {deafening},
             {earsplitting}, {roaring}, {thundery}]
     2: extremely ominous; "world events of thunderous import"