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 "Take that, sucker!" to a rude person. So.
Some times things are said that hurt. They may not have made any difference to you, but they do to the person you spoke them to. This can also work the other way. Encouraging words can stick with someone as well. Choose your words wisely