underhand vs underarm

underhand and underarm both are adjectives.

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

As adjectives, underhand and underarm are synonyms defined as:

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

Other synonyms of underhand include underhanded.

As adverbs, underhand and underarm are synonyms defined as:

underhand (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

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