What is nnid?

NNID stands for Nintendo Network ID. Here what they say about it on their site.

  • A Nintendo Network ID is an ID that grants access to various Nintendo Network services. Once you have a Nintendo Network ID, you can check its details either in User Settings on a linked Wii U console or in the Nintendo Network ID Settings portion of System Settings on a linked system in the Nintendo 3DS family.

    Currently, you can create a Nintendo Network ID using either a Wii U console or a system in the Nintendo 3DS family.