The Collaborative International Dictionary of English v.0.48:

Thunderous \Thun"der*ous\, a. [Written also thundrous.]
   1. Producing thunder. [R.]
      [1913 Webster]

            How he before the thunderous throne doth lie.
                                                  --Milton.
      [1913 Webster]

   2. Making a noise like thunder; sounding loud and deep;
      sonorous.
      [1913 Webster] -- Thun"der*ous*ly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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