Close female friend. According to Digital Underground (ref: 'Good Thing We're Rappin') the phrase is derived from the expression "Nucka" meaning more than simply "my nigga", but transcending this plainly overused phrase to apply to those that are very close friends. The addition of an antecedent "Sh" changes the word to the feminine, meaning "She-Nucka".

Friend , G Thing , Drankin Patna or just a negro you like to "kick it with"(in white terms)
Best friend..more than a black and more than a nigga
Your closest thug