'weathervane' originates from Old English elements: 'weather' (Old English 'weder') and 'vane' (Old English 'fana' or 'fæn(e)'), where 'weder' meant 'wind, storm' and 'fana' meant 'flag or banner'.

'weathervane' developed in Middle English as a compound (e.g. forms like 'weder-vane') and eventually became the modern English 'weathervane'.

Initially it meant 'a vane or banner used to show the direction of the wind'; over time it retained that literal sense and also acquired a figurative sense meaning 'a person who changes opinions according to the prevailing trend'.