Rue is a noun that means deep sorrow or regret, usually over something that cannot be changed. It is often used in the phrase 'RUE the day, ' which means to regret something that has already happened. The phrase 'regret bitterly' implies that the regret is intense and overwhelming. Therefore, the crossword solutions for the term REGRET BITTERLY can be summarized as a deep and intense sorrow or regret, usually over something that cannot be changed.