Slang for Ninja Kitty or Ninja Cat. Can also be used to describe a girl with epic awesome moves.
Originated from the series Naruto in which animal summons begin with 'Nin'
Use began at Dragon Con 06 during which a Naruto fangirl created it and started using it as her name, and famously continues to use it today.
"Did you see her beat up that guy?"
"Yeah man! That was a total Nin-Kitty!"