c. 1300, astonien, from Old French estoner "to stun, daze, deafen, astound," from Vulgar Latin *extonare, from Latin ex- "out" + tonare "to thunder" (see thunder); so, literally "to leave someone thunderstruck." The modern form (influenced by English verbs in -ish, such as distinguish, diminish) is attested from c. 1530.
No wonder is thogh that she were astoned [Chaucer, "Clerk's Tale"]