astound: [17] Astound, astonish, and stun all come ultimately from the same origin: a Vulgar Latin verb *extonāre, which literally meant something like ‘leave someone thunderstruck’ (it was formed from the Latin verb tonāre ‘thunder’). This became Old French estoner, which had three offshoots in English: it was borrowed into Middle English in the 13th century as astone or astun, and immediately lost its initial a, producing a form stun; then in the 15th century, in Scotland originally, it had the suffix -ish grafted on to it, producing astonish; and finally in the 17th century its past participle, astoned or, as it was also spelled, astound, formed the basis of a new verb. => astonish, stun
astound (v.)
mid-15c., from Middle English astouned, astoned (c. 1300), past participle of astonen, astonien "to stun" (see astonish), with more of the original sense of Vulgar Latin *extonare. Related: Astounded; astounding.