flag so or sth down meaning

[American slang]
to signal or wave, indicating that someone should stop.
  Please go out and f lag a taxi down. I'll be right out.
  She went to flag down a taxi.

• flag down:    Verb: flag down&nb ...
• belt so or sth down:    [American slang]to ...
• bind so or sth down:    [American slang]to ...