In a fluttering state; trembling, waving, or moving with quick irregular motions.
From 'a-' (in the state of) + 'flutter,' where 'flutter' comes from Old English 'floterian,' of imitative origin describing rapid light movements.
The phrase 'all aflutter' survives in modern English to describe excitement—it's one of the last common uses of this 'a-' prefix that was once used for hundreds of words in Middle English.
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