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".
Person A: Are you busy today?
Person B: Mmm... nah.
Person A: Do you want to go get dinner?
Person B: Mmm... nah.
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).