underachiever

un·der·achiev·er

noun \ˌən-dər-ə-ˈchē-vər\

Definition of UNDERACHIEVER

: a person (as a student) who fails to achieve his or her potential or does not do as well as expected

—un·der·achieve \-ˈchēv\ intransitive verb, un·der·achieved un·der·achiev·ing

—un·der·achieve·ment \-mənt\ noun