From WordNet (r) 3.0 (2006) :
below the belt
adv 1: in an unfair manner; "they dealt with him unfairly";
"their accusations hit below the belt" [syn: unfairly,
below the belt] [ant: clean, fair, fairly]
adj 1: disregarding the rules (from the notion of an illegal low
blow in boxing); "her accusations were below the belt"