Not a word for the faint-hearted, eunuch sounds like "you nick," and if you castrate a man you make him a eunuch by...uh...nicking the testicles with a knife or scalpel and removing them.
Eunuch comes from the Greek eunoukhos, meaning "bedroom guard." In history a eunuch was a man whose job was to guard the bedroom door of a royal woman to protect her. These guards were castrated to make them less threatening to the women they guarded. Later, some men underwent castration so their high voices wouldn't deepen during puberty, and these eunuch singers became famous in opera houses.