If a 3 year old tells you they want to cut off a body part for any freakin reason you take them to a doctor. The have mental problems.
Oliver Sacks, beloved by libs (although I think he makes up stories - and he has been called a fantasist) has written extensively about people who dissociate a limb. It may very well be a form of mental illness or some other kind of illness. But in today’s crazy world, that means we saw off the limb.