'atwitter' originates from the prefix 'a-' (meaning 'in a state of') combined with 'twitter', an imitative verb for rapid, high-pitched sounds.

'twitter' arose in Middle to Early Modern English as an imitative word for small, quick sounds (like birds' chirps); the prefix 'a-' (from Old English and Middle English use meaning 'in' or 'on') was attached in Early Modern English to form 'atwitter'.

Initially it evoked the literal sense of 'making small twittering sounds' or being 'like twittering birds' (i.e., visibly fluttering or agitated); over time it came to be used figuratively for human states of nervous or eager excitement and largely retains that figurative sense.