Clap the erasers

1) (common)To clap 2 chalk board erasers together to clean them and create a cloud of chalk dust, often fun when students are board.
2) (slang) To clap dem cheeks of the class slut.

When I went to school in the 30’s, at the end of every day a couple of students were assigned to knock that day’s build up of chalk dust from writing on the blackboard. So, they would go outside and clap two erasers together for a couple of minutes to knock out the chalk dust.