Likely aback-formation fromastoned, either apartial calque ofOld Frenchestoné(“stunned”), the past participle ofestoner(“to stun”), with the Old French participial suffix-é replaced with-ed and prefixed witha- or less likely, apartial calque of an unattested*astoné,*atoné; compareMiddle Frenchattoné(“shaken”) andOrléanaisattoner(“to be worried, to want to know”). Bysurface analysis,a- +stonen(“to stun”).
astonen (third-person singular simple presentastoneth,present participleastonende,astonynge,first-/third-person singular past indicative and past participleastoned)(uncommon)
1 Sometimes used as a formal 2nd-person singular.