underachiever means one who underachieves, i.e. performs less well than expected. Lexicurio rates it Sui generis — a strength score of 86 out of 100.
Etymology
From underachieve + -er or under- + achiever.
noun
- 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.”