Nah in most cultures means "No". In my culture, it means an assuring "Here, take it from me". Only family and close friends use it with each other. With a change of tone, it could also mean a dismissive "That that, sucker!" to a rude person.
I remember driving past the fire house with Vicky and Amy and Vicky always saying "Salute" and we all threw up the middle finger. lol I love the simplicity of a middle finger, it is so simple but says soooo much!