below the belt

adjectives

disregarding the rules (from the notion of an illegal low blow in boxing)
"her accusations were below the belt"
below the belt
is similar to
not fair; marked by injustice or partiality or deception
"used unfair methods"; "it was an unfair trial"; "took an unfair advantage"
unfair, unjust
domain category
fighting with the fists
boxing, fisticuffs, pugilism

adverbs

in an unfair manner
"they dealt with him unfairly"; "their accusations hit below the belt"
below the belt, unfairly