The Iron Butt is a machine that replaces a persons behind after the person badly injures it too many times. Not much is known about this machine, other than the fact that it itches (as stated by Tom).
 Machine details
The machine seems to be a spoof of a cast and the Iron Lung. The evidence of this is when Tom says that it itches. Also, in "I Had an Accident," SpongeBob was warned that he would get the machine if he gets another bad butt injury. The machine has a bunch of contraptions like sirens and train whistles. "If you did not use it, you are a very lucky person because you did not use the machine. You had better not break your butt again!" Also stated by Dr. Gill Gilliam, "a better way that is less painful than using the Iron Butt is STAPLING the entire butt back to your bottom in normal good shape."
 People who have used the Iron Butt
The only known person who has used the Iron Butt is Tom, no one else in the series has used it.
Not to be confused with the move, Iron Butt, from Krusty Krushers.