underhandedly vs underhand

underhandedly is not an adjective while underhand is an adjective.

Word	Noun	Adjective	Verb	Adverb
underhandedly	No	No	No	Yes
underhand	No	Yes	No	Yes

As adverbs, underhandedly and underhand are synonyms defined as:

underhandedly (adjective)	underhand (adjective)
	with hand brought forward and up from below shoulder level
	marked by deception

underhandedly (adverb)	underhand (adverb)
slyly and secretly	with the hand swung below shoulder level
	slyly and secretly