underarm vs underhand

underarm and underhand both are adjectives.

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

As adjectives, underarm and underhand are synonyms defined as:

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

Other synonyms of underarm include underhanded.

As adverbs, underarm and underhand are synonyms defined as:

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