It's from the IT novel by Stephen King. Here's the context:
Chris Unwin pushed him backward and Hagarty landed in a teeth-rattling
heap on the sidewalk.
What does it mean to land in a teeth-rattling heap?
What does “teeth-rattling heap” mean
expressionsmeaning
Best Answer
Because Hagarty was pushed over backwards, the fall was awkward and uncontrolled, leading to an ungainly posture on the ground. The force and shock of the fall made Hagarty's jaw open and close, with the teeth knocking against each other, causing a rattling sound.
The teeth-rattling heap condenses this into a short expressive phrase.