Wordnet 3.0
NOUN (2)
1. 
 an unnecessarily aggressive and unfair remark directed at a defenseless person; 
2. 
 an illegal and unsportsmanlike act of unnecessary violence; 
- Example: "he called a penalty on them when the lineman took a cheap shot at the quarterback"
WordNet (r) 3.0 (2006):
cheap shot
    n 1: an unnecessarily aggressive and unfair remark directed at a
         defenseless person
    2: an illegal and unsportsmanlike act of unnecessary violence;
       "he called a penalty on them when the lineman took a cheap
       shot at the quarterback"