below_the_belt
1. [ adjective ] disregarding the rules (from the notion of an illegal low blow in boxing)
Examples:

"her accusations were below the belt"

Related terms: boxing unfair
2. [ adverb ] in an unfair manner
Synonyms: unfairly
Examples:

"they dealt with him unfairly" "their accusations hit below the belt"

Related terms: fairly unfair unfair