low blow

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

• noun Sports An illegal blow in boxing that lands below the opponent's waist.
• noun An unscrupulous attack; an insult.

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

• noun boxing An unfair or illegal blow that lands below the opponent’s waist; a groin attack.
• noun by extension A rhetorical attack that is considered unfair or unscrupulous.

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

• noun unscrupulous abuse