'fearsome' originates from Old English, specifically the word 'fǣrsum,' where 'fǣr' meant 'sudden danger' and '-sum' was a suffix forming adjectives.

'fǣrsum' changed from Old English word 'fǣrsum' and eventually became the modern English word 'fearsome'.

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