Drop a dime

Today's phrase is "to drop a dime." It means to rat someone out to the police: 

 "That same unmarked car has been parked across the street for two hours. I'm beginning to think someone dropped a dime on us." 

The image references the act of dropping a dime into a pay phone, ideally on a gritty side street in a 1920s hardboiled detective novel. If you're ever privy to snitching, be sure to use this phrase to convey your familiarity with the criminal underworld with effortless cool.