willful and permanent crippling, mutilation, or disfigurement of any part of another's body; also, the crime of engaging in mayhem
Merriam-Webster's Dictionary of Law ©1996
This term falls under the fun category of words which have a specific technical meaning but are applied metaphorically in common usage. These are the words that mess with someone's head if they get used to the technical usage and then hear people throw them around where they don't really apply, e.g.,
"The rowdy youths did all kinds of mayhem."
"But I thought you said they only spray painted some mailboxes and broke a few windows."