Source : Webster's Revised Unabridged Dictionary (1913)

Atheistic \A`the*is"tic\, Atheistical \A`the*is"tic*al\, a.
   1. Pertaining to, implying, or containing, atheism; --
      applied to things; as, atheistic doctrines, opinions, or
      books.

            Atheistical explications of natural effects.
                                                  --Barrow.

   2. Disbelieving the existence of a God; impious; godless; --
      applied to persons; as, an atheistic writer. --
      {A`the*is"tic*al*ly}, adv. -- {A`the*is"tic*al*ness}, n.

Source : WordNet®

atheistic
     adj 1: rejecting any belief in gods [syn: {atheistical}, {unbelieving}]
     2: related to or characterized by or given to atheism; "atheist
        leanings" [syn: {atheist}, {atheistical}]