'southbound' originates from the combination of 'south' and 'bound', where 'south' refers to the cardinal direction and 'bound' means 'moving towards'.

'southbound' evolved from the Old English word 'sūþ' for 'south' and the Middle English 'bounden', eventually forming the modern English word 'southbound'.

Initially, it meant 'moving towards the south', and this meaning has largely remained the same in modern usage.