cheap shot

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

• noun An unfair or unsporting verbal attack on a vulnerable target.

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

• noun A deliberate, unsportsmanlike, and potentially illegal act of aggression against an opponent.
• noun by analogy A remark that unfairly disparages an obvious vulnerability of the target.

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

• noun an illegal and unsportsmanlike act of unnecessary violence
• noun an unnecessarily aggressive and unfair remark directed at a defenseless person