athletics vs underarm

athletics is a noun but underarm is not a noun.

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

athletics (noun)	underarm (noun)
an active diversion requiring physical exertion and competition
participation in sports events as an extracurricular activity
a contest between athletes

athletics (adjective)	underarm (adjective)
	with hand brought forward and up from below shoulder level

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