'afoot' originates from Middle English, specifically the phrase 'on foot', where 'a-' meant 'on' and 'foot' referred to the act of walking.

'on foot' transformed into the Middle English word 'afoote', and eventually became the modern English word 'afoot'.

Initially, it meant 'on foot or walking', but over time it evolved into its current meaning of 'in progress or happening'.