drop a dime

To snitch or inform on someone to a person of higher authority. Originally street slang for informing to police, it refers to the old price of using a public payphone (10 cents). Primarily heard in US, South Africa. Timmy is such a teacher's pet, always ready to drop a dime on any of the other students he thinks are misbehaving. He knew he was facing 10 years in prison, so he agreed to drop a dime or two to police in exchange for a lighter sentence.

Inform on or betray someone, as in No one can cheat in this class-someone's bound to drop a dime and tell the teacher. This expression, alluding to the ten-cent coin long used for making a telephone call, originated as underworld slang for phoning the police to inform on a criminal and occasionally is extended to any kind of betrayal. [1960s]

To make a telephone call, especially to the police to inform on or betray someone.

To snitch, to betray. Once upon a time, pay phones in enclosed booths could be found on most urban streets as well as in other public areas. Vandalism was rare, so the phones worked, and equally surprising to us today, local calls cost a dime. A person who wanted to report something to the authorities that he or she didn't want anyone to overhear and didn't want the call traced located a secluded phone booth and deposited ten cents. That's the dime that was dropped down the coin slot. The dime-dropper took a big chance, because if the droppee found out, the rat stood a good chance of being exterminated.