The term 'flag down' is an idiom that means to STOP or attract attention. The crossword solutions for this term are STOP (4 letters), WAVE (4 letters), HAIL (4 letters), and ACAB (4 letters). The solution STOP suggests that one should come to a halt in order to flag down someone or something. The solution WAVE implies that one should use hand gestures to get the attention of the person or object they are trying to flag down. The solution HAIL suggests that one should call out to the person or object they are trying to flag down. Finally, the solution ACAB stands for 'All Cops Are Bastards, ' which is a slogan used by some people to express their dissatisfaction with law enforcement.