Top Definition
a sister who is also a best friend, or a friend who is closer to you than a friend and is more like a sister.
My sister, Kim, is also my best friend; she is my sister-biffle!
I love my friend Kristin so much, we are such good friends that she is more like a sister to me; she is my sister-biffle!
by BAD1484 May 13, 2010

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