The term 'bitterly regret' can be broken down into five crossword solutions: RUE (3 letters), RUED (4 letters), RAIL (4 letters), RAILED (6 letters), and INVEIGH (7 letters). RUE is a verb meaning to feel deep sorrow or regret for something done or experienced in the past. RUED is the past tense of the verb RUE, meaning to have felt deep sorrow or regret for something done or experienced in the past. RAIL is a verb meaning to criticize someone or something severely and bitterly. RAILED is the past tense of the verb RAIL, meaning to have criticized someone or something severely and bitterly. INVEIGH is a verb meaning to express strong disapproval or criticism of someone or something. Together, these five words describe the feeling of deeply regretting something and expressing strong criticism or disapproval of it.