Stunning

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Middle English: shortening of Old French estoner ‘astonish’.

wiktionary

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From Middle English stunnyng, stunnynge, stounyng, equivalent to stun +‎ -ing.

From Middle English stunnyng, stonyng (also stoniynge, stonyynge), equivalent to stun +‎ -ing.

etymonline

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stunning (adj.)

1660s, "dazzling," present-participle adjective from stun (v.). Popularized for "splendid, excellent" c. 1849. Related: Stunningly.