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1.
Person who fails to achieve his or her potential. Used often when refering to students.
My kid is an underachiever. He's not an idiot though.
by jakub777 October 24, 2008
 
2.
A female whose stomach petrudes farther outward from her body than her breasts.
Look at that fat chick with tiny boobs. What an under achiever!
by will_minnesota July 09, 2006
 
3.
The opposite of:

overachiever
44 up, 15 down
Someone who is sooo much better than you that it pisses you off.

I hate that fucking overachiever!

by Blah Aug 8, 2003 email it
Yeah so what if all I did was copy and paste the definition of overachiever. I'm a true underachiever.
by rappathered March 25, 2008
 
4.
A person who does not perform to one's expectations.

A person who disappoints.
Dr. Manmohan Singh Indian Prime Minister
as described very well by Time Magazine. He was depicted on the front page as underachiever.
by Shobit September 29, 2012