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To be atracted to or engage in sexual relations with a close blood relative such as a cousin or a sister. Taking from the relationship of the twins Jaime and Cercie Lannister of the popular book seires "A Song Of Ice and Fire".
Person 1: My cousin is so hot and she totaly wants me

Person2: Wow dude are you guys going Lannister on me?
by Tyron123 August 16, 2011
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