Free. You can use it to replace "de graça" (for free), in an expression.

By force.

Same as hit the bull's eyes. Like when you are on the 21st floor, and throws an egg on that annoying neighbor while he is walking his dog. You scream "Na mosca!".

Not a sound. What sometimes your mother tells you when she's on the phone and doesn't want to listen another sound.

"Nó" stands for "Minha Nossa Senhora!", "Nossa Senhora!", "Minha Nossa Senhora Do Perpétuo Socorro!", or simply "Nossa!".

Probably you will hear Mineiros using this expression, or people from the countryside from São Paulo.