'Not good' is just the ordinary negative of 'good'. If you're talking about a film or a book or a movie and you say is 'It's not good', it may be bad, or maybe it's simply mediocre or average - we don't know. All this tells you is that you don't think it's good. 'It's no good' is an idiomatic expression. This isn't simply the negative of good. This idiom means that a thing is totally pointless or worthless. It can refer to an object. For example, a device that is completely broken and unfixable, or that doesn't do its job at all can be 'no good' - it's worthless and may as well thrown away. A person who is 'no good' at something it means they have no ability at all. If you hire someone as a translator and you find that this person is 'no good at languages', they are no use to you and you should fire them. 'No good' can also refer to an action or pursuit. For example, if you have tried and tried to do something and you are convinced that it's impossible, you'd say 'It's no good. I can't do it.' This means that it's not worth trying any more and you should stop. I hope that helps.

however " no good" isn't the only word we negate that way: It's no fun: There is no way I' m doing that :: I have no idea: There is no chance of success:: no dice: It is important to note that with some of these the expressions it is imphatic form of another phrase . No idea is emphatic for "I don't know."