clapper
like the clappers very fast or very hard. British informal
☞ Clappers may refer to the striking part of a bell, or it may refer to a device in a mill for striking or shaking the hopper in order to make the grain move down to the millstones. The phrase like the clappers developed as mid-20th-century RAF slang, and is sometimes found in the form like the clappers of hell.
1992Jeff TorringtonSwing Hammer Swing! Why should a hearse be going like the clappers through the streets of Glasgow at this time of night?