underdress vs garment

underdress is not a noun while garment is a noun.

Word	Noun	Adjective	Verb	Adverb
underdress	No	No	Yes	No
garment	Yes	No	Yes	No

As verbs, garment is a hypernym of underdress; that is, garment is a word with a broader meaning than underdress:

Other hypernyms of underdress include apparel, clothe, dress, enclothe, fit out, garb, habilitate, raiment, tog.

underdress (noun)	garment (noun)
	an article of clothing

underdress (verb)	garment (verb)
dress informally and casually	provide with clothes or put clothes on
dress without sufficient warmth