'stunnedly' is a modern adverb formed by adding the adverbial suffix '-ly' to the past participle 'stunned', which comes from the verb 'stun' (English).

'stun' developed in Middle English as 'stunen' (or similar spellings) and later appeared as 'stun'/'stunned' in modern English; adding '-ly' produced 'stunnedly' as an adverbial form.

Originally the verb 'stun' meant 'to strike senseless or astonish'; over time the core idea of causing temporary shock or amazement has remained, and 'stunnedly' expresses doing something in that shocked manner.