1829, in pugilism, agent noun from stun. Meaning "beautiful woman" attested by 1848 on notion of "one who astounds or amazes."
Entries linking to stunner
stun v.
early 14c., "to daze or render unconscious" (from a blow, powerful emotion, etc.), probably a shortening of Old French estoner "to stun" (see astonish). Related: Stunned; stunning.