underachiever vs nonachiever

underachiever and nonachiever both are nouns.

Word	Noun	Adjective	Verb	Adverb
underachiever	Yes	No	No	No
nonachiever	Yes	No	No	No

As nouns, underachiever and nonachiever are synonyms defined as:

underachiever and nonachiever: a student who does not perform as well as expected or as well as the IQ indicates

Other synonyms of underachiever include underperformer.

underachiever (noun)	nonachiever (noun)
a student who does not perform as well as expected or as well as the IQ indicates	a student who does not perform as well as expected or as well as the IQ indicates