underarm vs underhanded

underarm and underhanded both are adjectives.

Word	Noun	Adjective	Verb	Adverb
underarm	No	Yes	No	Yes
underhanded	No	Yes	No	No

As adjectives, underarm and underhanded are synonyms defined as:

underarm and underhanded: with hand brought forward and up from below shoulder level

Other synonyms of underarm include underhand.

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

underarm (adverb)	underhanded (adverb)
with the hand swung below shoulder level