'astir' originates from Middle English, specifically the word 'a-stir', where 'a-' meant 'on' or 'in' and 'stir' meant 'to move'.

'astir' changed from the Middle English form 'a-stir' and was carried into Modern English with little alteration to become 'astir'.

Initially it meant 'on the move' or 'in motion', and over time it evolved into the current sense of 'in a state of movement or excited activity'.