dirty word

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An obscene or vulgar word.
2. n. A word, expression, or concept viewed as odious or intolerable: a feisty campaigner to whom "give up” was a dirty word.

Wiktionary

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

WordNet 3.0

1. n. a word that is considered to be unmentionable
2. n. an offensive or indecent word or phrase