pickpocket
扒手

explanation :
 A pickpocket is a kind of thief who wanders city streets and tries to steal wallets and purses. The idiom is easy to remember since it means exactly what it says.  People are usually warned about pickpockets before they go to busy cities.  

e.g.: If you're going to Rome, watch out for your wallet.  I've heard there are a lot of pickpockets in the city.