A fusion of the words "no" and "yeah". Used to annoy people who ask too many "yes" or "no" questions; or to say "yes" and "no" to controversial topics to avoid upsetting anyone.
Question: should marijuana be legalized to the same capacity as alcohol?
Answer : Nah
Question: did you put the bacon in the freezer yet?
Answer: Nah
person 1:Hey dude is this the stop.
person 2: Nah you don't say.
in this context it is used to as a way to say no you don't say it is used in that way so that the person who has received that comment realises that they are on the correct stop