flag down
To wave one's arms to attract someone's attention, usually to get them to stop moving. A noun or pronoun can be used between "flag" and "down." I was finally able to flag down a passing motorist to help with our stalled car. It'll take awhile to flag a taxi down. Why don't we just walk to the museum?
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
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.
flag down
v. 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.