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. The unusual form is perhaps because the past participle was so much more common it came to be taken for the infinitive, or/and by the same pattern which produced round (v.) from round (adj.), or by the intrusion of an unetymological -d as in sound (n.1). Related: Astounded; astounding.
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