Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1300, "to daze or render unconscious" (from a blow, powerful emotion, etc.), probably a shortening of Old French estoner "to stun" (see astonish). Stunning popularized for "splendid, excellent" c.1849.
Attractive; impressive: Isn't that a stunning little dress? (1849+)