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1.
Female version of "Nephew", individuals using niece/nephew try to get people off of using the n-word. Means the same (ie] sister/brother), but in a less aggressive/derogatory sense.
"That niece has crazy booty..."
by Vitamin T December 15, 2005
 
2.
free babysitter.a person who chooses to watch children against their freewill
hey wanna go to the club?.. sure let me call, my niece she is my free babysitter!
by overlord exodus February 19, 2008
 
3.
an expression to convey happiness and recognition of a favorable event, while also highlighting the misspelling of the original word "Nice." The expression is usually reserved for ironic situations since the word is recognized as a misspelling.
Roland : Hey Monica, you just won the Mega Millions lottery.

Monica : Niece!!!
by Rolando Montoya May 17, 2012