The Iron Butt is a device was only mentioned and shown in the episode "I Had an Accident."
It is a large gray machine with many wires, gears, pipes, and other things sticking out of it.
Role in episode
It is a machine that doctors use in Bikini Bottom if a patient's butt is severely injured. Tom once had to use one. SpongeBob fears the Iron Butt greatly. Seeing the Iron Butt resulted in SpongeBob wanting to stay indoors for his entire life so he is safe.
The Iron Butt is a parody of a device called the Iron Lung, which is used for breathing problems because of Polio.