below the belt vs unfairly

below the belt is an adjective but unfairly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
below the belt	No	Yes	No	Yes
unfairly	No	No	No	Yes

As adverbs, below the belt and unfairly are synonyms defined as:

below the belt (adjective)	unfairly (adjective)
disregarding the rules (from the notion of an illegal low blow in boxing)

below the belt (adverb)	unfairly (adverb)
in an unfair manner	in an unfair manner