Goneril is the eldest daughter in Shakespeare's King Lear.
Goneril treats her father well, until he becomes old, and gives her and her husband a plot of land and a lot of money. Then she starts treating him really crappily.
A girl, or guy, who is described as a Goneril, is someone who abuses and takes advantage of those who love him/her the most.
"My brother's daughter turned out to be such a Goneril. We haven't spoken with her since he died."