'bitterly' originates from Old English, specifically the word 'biterlice,' where 'biter' meant 'sharp or cutting.'

'biterlice' transformed into the Middle English word 'biterliche,' and eventually became the modern English word 'bitterly.'

Initially, it meant 'in a sharp or cutting manner,' but over time it evolved into its current meaning of 'with strong feelings of resentment or disappointment.'