What is "safety coffin"

n. Any coffin specially designed for the occupant to leave it in case of premature burial, such as by warning other people to help him.

A safety coffin or security coffin is a coffin fitted with a mechanism to prevent premature burial or allow the occupant to signal that they have been buried alive. A large number of designs for safety coffins were patented during the 18th and 19th centuries and variations on the idea are still available today.