"your junk" actually has another meaning now. google Brett Favre junk and see what you get (don't search images) in USA, I hear, "that person is trashy, or is trash," others too but I will not say

You wouldn't just say "You're junk". Instead you would say "You're a piece of junk" or more commonly "You're trashy". Remember, these are all negative statements!

I'm not sure about other parts of the country, but on the west coast USA that would sound weird. Native speakers would more likely say something like, "you're trashy". But it's really talking about how much social class they are or aren't showing...

No, not on its own. You can say that someone is a 'piece of junk'.