Ixney on the whatnay?

[*ashie*]

What does ixney on the *insert text* nay mean?!? I understand that it means be quiet about this topic but where did it come from and why?!?

[Clanad]

It's Pig Latin... a type of coded language, probably made up by children at sometime in the dim past.  One simply removes the first letter of each word, places it at the end of the word and adds "ay"... hence your "ixnay is actually nix, or delete or be quiet about...

[Kingaroo]

As Clanad says, kids used to use Pig Latin to  transmit a secret message.  Nobody says "Nix on that" anymore, but it used to mean "Cut it out."

Note that the second word is not always followed by Nay, as in your question -- it depends what sound the word being altered starts with.

Well known example: in the Disney cartoon Lion King, Simba tells Zazu that hyenas are stupid, and Zazu notices the hyenas listening.  He says to Simba, "Ix-nay on the upid-stay!" 

....ig-pay atin-lay ocks-ray!