Top definition
A girl who has an affair with a man already involved with someone else, and has been for over two years. He just uses this mistake girl to prove to himself how much he loves he loves his significant other...and after the love-making with the mistake girl, he goes back to his woman with a completely new outlook on their relationship. Nobody but the mistake girl gets hurt in the end because she thought they had something special when after all she was just being used.
"When we weren't in love anymore, I just used this mistake girl to make myself realize how much I love you."

"Don't worry, she was only the mistake girl. It didn't mean anything."
by aamandadam December 07, 2008
Get the mug
Get a Mistake Girl mug for your mate Beatrix.