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"