The term 'Regrets Bitterly' can be summarized by the four-letter word 'RUES'. This word is derived from the Latin verb 'ruo', which means 'to regret'. Therefore, 'RUES' can be used to describe someone who deeply regrets something they have done or experienced. This regret is often accompanied by a feeling of bitterness, as the person may feel that they have been wronged or that their actions have had negative consequences.