underachiever

noun

Etymology: From underachieve + -er or under- + achiever.

1. One who underachieves, i.e. performs less well than expected.

   “Woody Allen has provided what is probably the best theological answer to the problem of evil: God is not evil; He is just an underachiever.”

   “Bart: (reading his permanent record) "Underachiever and proud of it." How old is this thing?”