Nah kid
A soft way to tell someone to go away. It's like you're being polite but you're also being a bit of a jerk.
Nah kid, I'm not going to that restaurant. I don't like the food.
Nah kid, I'm not helping you with your math. I don't want to fail with you.
Nah kid, I'm not coming to your birthday. I don't want to be stuck with you.