The Collaborative International Dictionary of English v.0.48:

Stunner \Stun"ner\, n.
   1. One who, or that which, stuns.
      [1913 Webster]

   2. Something striking or amazing in quality; something of
      extraordinary excellence. [Slang] --Thackeray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stunner
    n 1: a very attractive or seductive looking woman [syn:
         smasher, stunner, knockout, beauty, ravisher,
         sweetheart, peach, lulu, looker, mantrap, dish]
    2: an unexpected and amazing event; "the stunner was what
       happened on Saturday"