Nah: (Nn-ahh) Noun. A word commonly used as a replacement for "no", "nope", "shut up", or "no, don't give me that food, I don't want to get fat". But mostly "no".
This is the shortage of "na huj" (на ХУЙ) - "nah", wich means "fuck no" or "I dont care". Most popular slang word in russian language between young ppl for long time (ages).