Possibly of
East London origin, typically, meaning a weak polite form of 'no' but often 'Nah' is a negative when
the speaker is certain that the listener is incorrect and both know that fact for certain but the listener is
doublethinking themselves into belief.
"My
boyfriend wants to marry me but he isn't ready to get married.. we both have houses to sell before we move on.. my
boyfriend needs to sort his shed out before we can get married and support each other but for now he's put me
on the game to help out.. I do have my 'hero' and he wants to marry me"
"Nah"