From Longman Dictionary of Contemporary Englishflag somebody/something ↔ down phrasal verbSTOP MOVINGto make the driver of a vehicle stop by waving at them I flagged down a taxi. → flag→ See Verb table
Examples from the Corpus
flag down• How about pantyhose made out of the flag?• John Macgill was at Edinburgh's Dreghorn barracks to see the flags come down.• Two young women flag me down.• After about two blocks, we flagged the driver down and climbed aboard.• He flagged a waiter down and grabbed two drinks from the silver tray.• He saw the red flag come down for the final time over Red Square in Moscow.• If some one tries to flag you down, make sure it is a genuine emergency before getting out of your car.• The official explanation was that a finish flag had blown down on to the mechanism and the timing had run on. 