'fearful' originates from Old English, specifically the word 'fǣrfull', where 'fǣr' meant 'danger' or 'sudden calamity' and '-full' meant 'full of'.

'fǣrfull' transformed into the Middle English word 'ferful', and eventually became the modern English word 'fearful'.

Initially, it meant 'full of danger or sudden calamity', but over time it evolved into its current meaning of 'feeling or causing fear'.