The English language has many strange and obscure words, ranging from one's with rather obscure meanings such as - jumentous - to smell of horse urine, and one's that have more practical uses such as - callipygean - Having well shaped buttocks. One often associated with me.
The word you're referring to I had never heard used in dialogue before but would give a nifty score on Scrabble if you're low on letters.
Nix - pronounced 'Niks',
It's noun - Nothing
It's adverb - No (which is what you're referring to)
Interjection
When used as an interjection it's an exclamation, a warning.
"Nix, the police!"
Verb
When used as a verb (used with object) it means to object/to refuse to agree to something/prohibit.
"to nix the assignment"
It's origin is German 18th Century.
If you'd like to learn more interesting and unusual words please check this out!
users.tinyonline.co.uk/gswithenbank/unuwords.htm