When you impregnate your sister in-law and fool your brother into raising him as his own.
Person 1: I took my nephewson to the museum this weekend.
Person 2: Have you told your brother yet that its not his son?
Person 1: Nope.
by Spiderbat September 21, 2011
A boy that was a result of artifical insemination but was born to the partner of a sibling.
Coined by William from "Girlfriends" to explain that his sister's son was born to her girlfriend who was inseminated by William. Usually comes up when William has to shop or participate in some activity. i.e., "I went to the park with my nephew son."
by Liz Kupcha October 02, 2008

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