'astound' originates from Old French, specifically the word 'estoner' (or Old French 'estoner'/Middle English 'astounen'), ultimately from Latin 'extonare', where 'ex-' meant 'out' and 'tonare' meant 'to thunder'.

'astound' changed from Middle English 'astounen' (influenced by Old French 'estoner') and eventually became the modern English word 'astound'; the adverbial form 'astoundingly' is formed from the adjective 'astounding' + adverbial suffix '-ly'.

Initially it carried the sense of being struck or overwhelmed (literally 'struck like thunder'); over time it evolved to mean 'to surprise or shock greatly', and 'astoundingly' now means 'in a way that causes great surprise or admiration'.