Top Definition
The female child of your bruncle/aunster. Your bruncle and/or aunster is your aunt/uncle and sibling at the same time, so their children, in this case the female, is your niesin; your sibling's daughter being your niece, and your aunt/uncle's daughter being your cousin. Therefore the end result is biologically your niesin.
My niesin Sally is such a cutie pie, she's my niece and my cousin at the same time.
by wnywenke December 24, 2009

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