If you strapped down an adult man by the legs and arms, naked, against his will, then cut off part of his penis without painkiller and without consent, what would the charge be?
(Interested in USA and UK mainly)
For the US, there are 50 technically distinct answers since ordinary crimes are defined at the state level. However, on this point states will be broadly quite similar. The "strap down" part is the Class C felony false imprisonment (knowingly restraining another person). Nakedness does not add anything. Cutting anything off is the Class A felony first degree assault (assault by means likely to produce great bodily harm). Great bodily harm includes bodily injury which causes a significant permanent loss of any bodily part. The law does not define what is "significant". Painkillers do not change what the crime is, nor does the specific body part. 1st degree assault covers a wide range of actions, since it also includes assaults that could have resulted in great bodily harm, but did not actually.
The penalty for a class A felony is a maximum of life imprisonment and $50,000 fine. The state's sentencing laws are complex, so without more details it is impossible to guess about minimum sentencing.
In the US, that kind of criminal law is a matter of state law, and the exact terms may vary from one state to another. Probably doing such a thing would involve multiple offenses. Cutting off any body part would surely be Aggravated Assault. It would probably also be some degree of Sexual Assault (the numbers very by state). Restraining the victim might be false imprisonment or kidnapping.The old common-law offense of maiming might also apply.
In the UK, this would probably be Grievous Bodily Harm, which carries a possible life sentence. It might be treated (either as a lesser charge with a greater chance of conviction or as a lesser included offence) as actual bodily harm, which may lead to a term of imprisonment (unspecified, but in practice short) or something less.