The big ouch, for a guy. It is the removal of a male's genitals, making him, well unable to perform as his penis and/or testicles.
Can also refer to a mannerism, in which the male is made to be useless, as a man, without the need for surgery.
Usually accomplished by some form of humiliation.
More often than not, that humiliation takes the form in some sexual connotation.
Furthermore, it refers to the gutting of some pieces of legislation, from their original form, to a less than forceful one. Usually involves the removal or watering down of penalties in legislation.
Today, the more social meaning is better known, rather than the actual surgical method, also known as castration.