underhanded vs underarm

underhanded and underarm both are adjectives.

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

As adjectives, underhanded and underarm are synonyms defined as:

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

Other synonyms of underhanded include underhand.

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

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