Wordnet 3.0
VERB (1)
1. 
 signal to stop; 
- Example: "Let's flag down a cab--it's starting to rain"- Example: "The policeman flagged down our car"
WordNet (r) 3.0 (2006):
flag down
    v 1: signal to stop; "Let's flag down a cab--it's starting to
         rain"; "The policeman flagged down our car"