dirty word

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An obscene or vulgar word.
• noun A word, expression, or concept viewed as odious or intolerable.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A word that is considered vulgar, not necessarily sexual in nature.
• noun idiomatic The name of a topic that a person does not like to hear or discuss.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a word that is considered to be unmentionable
• noun an offensive or indecent word or phrase