'passerby' originates from the combination of 'passer' and 'by', where 'passer' refers to someone who passes and 'by' indicates proximity.

'Passerby' evolved from the Middle English word 'passen' meaning 'to pass' and the Old English 'bi' meaning 'near'.

Initially, it meant 'someone passing by', and this meaning has largely remained the same in modern usage.