'astonishedly' originates from English, specifically formed from the adjective 'astonished' + the adverbial suffix '-ly', where 'astonished' is the past-participial/adjectival form of the verb 'astonish', and 'astonish' ultimately comes from Old French 'estoner' (Anglo-Norman) derived from Latin elements 'ex-' + 'tonare' ('to thunder').

'astonishedly' developed from Middle English/early modern English forms: Latin 'tonare' (to thunder) influenced Old French 'estoner'/'etoner', which became Anglo-Norman/Old French; this produced Middle English 'astonish' (to stun or amaze), then the past-participle/adjective 'astonished', and finally the adverb 'astonishedly' by addition of '-ly'.

Initially related to the idea of being struck or stunned (literally 'thunder-struck'), the sense shifted to 'to amaze or surprise', and the adverb now means 'in a manner showing astonishment'.