In a way that is shocking, surprising, or impressively beautiful and remarkable.
From 'stunning' (adjective) plus the adverbial suffix '-ly'; 'stun' comes from Old French 'estoner' and originally meant to daze or shock.
The word 'stunningly' has become increasingly common in entertainment criticism and marketing—linguists have noticed it's now often used to describe ordinary things, which shows how superlatives gradually lose their shock value as we use them more frequently.
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