The Collaborative International Dictionary of English v.0.48:

Whereat \Where*at"\, adv.
   [1913 Webster]
   1. At which; upon which; whereupon; -- used relatively.
      [1913 Webster]

            They vote; whereat his speech he thus renews.
                                                  --Milton.
      [1913 Webster]

            Whereat he was no less angry and ashamed than
            desirous to obey Zelmane.             --Sir P.
                                                  Sidney.
      [1913 Webster]

   2. At what; -- used interrogatively; as, whereat are you
      offended?
      [1913 Webster]