four-letter word

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

• noun Any of several short English words generally regarded as vulgar or obscene.

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

• noun A reference to any of several of the strongest English swearwords that are also four letters long, especially those composing the so-called seven dirty words.
• noun A swearword or any words considered to be taboo in a given scenario, (regardless of how many letters are actually in the word or words in question).

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

• noun any of several short English words (often having 4 letters) generally regarded as obscene or offensive