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1.
1. A weak or undependable member of a group. 2. A person regarded as timid or indecisive.
Jim's a fuckin' weak sister, he couldn't decide whether to shit or go blind.
by zboss June 10, 2007
 
2.
The lesser of two twins, regardless of sex. There is always a dominant twin.
Jesse may be a superior athlete but he is totally the Weak Sister.
by the_captain_awesome July 16, 2008