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Related to flag down: at first, Put the flags out
flag someone or something down
to signal or wave, indicating that someone should stop. Please go out and flag a taxi down. I'll be right out. She went to flag down a taxi.
flag down somebody/somethingalso flag somebody/something down
to signal someone or something to stop by waving A police officer flagged us down to check our vehicle registration and date of inspection.
Signal to stop, as in The police were flagging down all cars. This expression uses the verb flag in the sense of "catch the attention of, as by waving a flag," a usage dating from the mid-1800s; down was added in the first half of the 1900s.
To signal something or someone to stop: I flagged down a taxi when it started raining. When we ran out of gas, we flagged the police officer down to ask for help.