unfairly vs below the belt

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

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

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

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

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