the GNU version of the Collaborative International Dictionary of English
ad. At which; upon which; whereupon; -- used relatively.
ad. At what; -- used interrogatively; as, whereat are you offended?
The Century Dictionary and Cyclopedia
At what? as, whereat are you offended?
At which.
Word Usage
"On the lower shelves were enormous folios, on whose backs I spelled in black letters, 'Lightfoot Opera,' a title whereat I wondered, considering the bulk of the volumes."