D defs.my
Entry 2 senses Webster, 1913

Whereat

· Where·at · IPA /wɛəɹˈæt/
01 adv. At which; upon which; whereupon; -- used relatively.
  1. 1.
    At which; upon which; whereupon; -- used relatively.
    “They vote; whereat his speech he thus renews.” Milton.
    Whereat he was no less angry and ashamed than desirous to obey Zelmane.” Sir P. Sidney.
  2. 2.
    At what; -- used interrogatively; as, whereat are you offended?