The term 'bitterly complain' can be broken down into four crossword solutions: RAIL (4 letters), RAILED (6 letters), GROUSE (6 letters), and INVEIGH (7 letters). RAIL is a verb meaning to criticize or complain about something in an angry or forceful way. RAILED is the past tense of this verb, indicating that the criticism has already been expressed. GROUSE is another verb meaning to express dissatisfaction or annoyance. INVEIGH is a verb meaning to express strong disapproval or criticism. All four words are related to expressing discontent or displeasure in a strong manner.